by adding rescue skills
PADI Open Water, BSAC Ocean Diver and other equivalent basic qualifications do not properly prepare divers to dive unsupervised. The training agencies attach guarded comments such as under the supervision of... to these basic qualifications.
Supervision is essential while gaining experience. For experienced divers it can be nice for guidance and pointing out interesting bits of the dive, but relying on supervision for safety is not a comfortable long-term strategy.
There is always the chance that, just when you have a problem, your attentive and diligent dive guide is already busy helping out another diver. This scenario crops up often enough in real incidents.
The best way to ensure your own safety, and that of your buddy, is to learn enough to be able to look after yourselves.
The training that brings all this together is the rescue stuff. In the PADI system, this means completing Advanced Open Water and Rescue Diver courses. In the BSAC system, it means getting to Sports Diver, then doing a Practical Rescue Management course. These courses teach the skills necessary to handle any diver rescue; the same skills a dive guide or instructor provides.
Rescue courses are also a lot of fun, involving taking turns to do some horrendous over-acting as victims and participants in all sorts of imaginative diving-incident scenarios.
But it isnt just about the actual rescue process. These courses include a fair bit about general dive-planning and anticipation of potential problems.
Divers also get a chance to practise their own diving techniques. Simple things such as finning and buoyancy control must become subconscious skills when youre busy focusing on a rescue.
If your buoyancy isnt under control by the end of all this, its time to sign up for a buoyancy workshop.
Being trained to assess the situation, handle the rescue and manage the overall incident marks the line between depending on others for your safety and being in control.

and cover your insurance
You may have read about decompression illness incidents where divers on holiday had been deeper than the depths to which they were qualified, their insurance company disowning them as a result.
Basic qualifications rightly limit diving to 20m, but it is not uncommon for a guided dive to go deeper than this.
In the course of gaining rescue skills, a diver will also have gained qualifications to 30m (PADI AOW) or 35m (BSAC Sports Diver). Such limits should be enough for the foreseeable diving future, but even the best-intentioned dives in nice conditions can easily stray beyond these limits, so its worth collecting a qualification that goes a little further, if only to keep the travel insurance valid.
Depth and deep diving is not the objective in itself. Too many new divers develop an unhealthy depth fixation, pushing themselves deeper for the sake of it, and moving on to trimix as soon as they meet the minimum entry requirements.
Such depth-obsessed divers load themselves up with complicated dive plans and equipment. However, sometimes, despite their qualifications, they have not developed the breadth of basic experience they really need as a foundation.
At this stage, the aim should be to get a qualification that covers divers beyond the sort of diving they really want to do, then to dive well within that qualification while consolidating skills and experience.
Get a 40m qualification, but dive within 30m and allow for the odd transgression of a metre or two.
Suitable qualifications are PADI Deep Diver (40m), BSAC Dive Leader (50m) and Advanced Nitrox with Decompression Procedures (45m) from most technical agencies, though the latter two courses include far more than is really needed at this stage.

and get a bit technical
In these days of dive computers, many divers start to build up depth and decompression without really thinking about it.
A typical multi-level dive follows a reef or wall, slowly working down to whatever the maximum depth is, takes a brief look at whatever is there, then returns shallower as the dive continues, never quite getting a dive computer into decompression.
All too soon, a short stay at depth and no deco turns into letting a few minutes of stops accumulate and following the computer up, the stops disappearing on the shallow segment at the end of the dive. The accepted amount of decompression casually increases to the point at which divers make quite serious decompression dives for which they have never been properly trained.
Rather than letting decompression diving creep in through the back door, divers need to be trained for it and apply good practice from the outset.
BSAC Sports Diver and Dive Leader courses include planned deco stops, but to really learn about equipment and techniques for decompression diving and the associated risk/contingency planning, the best course is Advanced Nitrox with Decompression Procedures from one of the technical agencies.
Most basic training already includes an introduction to nitrox, but if it does not, the Advanced Nitrox side of this course takes care of anything a diver needs to know about using enriched air, and adds far more detail into the bargain.
This is not a course to rush into, but when you come home from a trip and one or two of the dives have crept past a minute or two of decompression, you should be doing an Advanced Nitrox with Decompression Procedures course. Even if the only thing you learn is why you shouldnt be doing it.

who really are experts
To get the most from a course, you need an instructor who both knows how to teach and also knows about what he or she is teaching.
When the course covers core diving skills, all diving instructors should know more than enough, but on the more specialised courses such expertise may be less apparent.
Underwater photography is a classic example of
a course with which divers are frequently disappointed. They sit through a classroom session with an instructor who knows little more than they do, then get sent off with a camera to take some pictures, followed by another classroom session to look at the results.
The instructor doesnt even get in the water with them. You could call it a camera try-dive, but calling it an underwater photography course is close to misrepresentation.
For a worthwhile course, you need an instructor who is also an underwater photographer. Or, better still, an underwater photographer who is also an instructor. And the instructor needs to work with the students in the water.
The same goes for other specialised skills such as underwater video, archaeology and marine biology. For video training, look for an active videographer who is also an instructor. For archaeological training, go to the Nautical Archaeology Society.
For marine biology, in the UK do an MCS/ Seasearch course, and in tropical water have a
short stay with an expedition group such as Coral Cay Conservation.
The same applies to advanced diving skills. To learn technical or rebreather diving, look for an instructor who actually does this sort of diving, rather than just teaches it. To learn to be a divemaster, find a instructor who does dive-guiding and the other bits of a divemasters job on a regular basis, rather than one who just teaches lots of divemaster courses.
To learn to be an instructor, seek out an instructor-trainer or course director who still teaches divers, rather than teaching instructor courses only.

for related skills
There are plenty of opportunities to enhance diving-related skills by taking courses outside diving.
These may not be diving-specific, but by this stage a diver should know enough to take the general information gained and apply it to a diving situation.
It also comes back to our previous tip, choosing specialist instructors who are experts.
Do you really need to learn from a diver For photographic training, perhaps a general photography course will come in useful. Many of the techniques are the same.
Similarly, first-aid courses taught by a diving instructor cover the minimum a diver needs to know, but there is so much more that can be learned on a more general first-aid course taught by a doctor, nurse or paramedic who has all the real front-line blood and guts experience.
For the chartwork side of dive planning, the RYA Yachtmaster Theory course is a good way of filling winter evenings, and it teaches much more about the academic side of navigation than any diving course does.

Use multiple sources
A mistake many divers make is to start with one instructor, school and agency, get on well
with them, then stick with them for all subsequent training.
No matter how good they are, and how big an effort they make to be open-minded and non-partisan, it remains just one view of things. Diving is much bigger and more complicated than that.
By all means, take the first few diving courses up to and including rescue from the same source. Consistency at such an early stage will help more than diversity. But when it comes to further training, a divers education and skills will be better served by injecting a bit of variety.
If you come from a club background, do a course with a school, and vice versa (though it can be harder to organise).
Dont fall into the easy and comfortable habit of sticking with one instructor. Look further afield and continue your training elsewhere, with a different instructor, school and agency, for varietys sake.
In the long term, the diversity of teaching style, opinions, and environment will make you a better diver.

with a regular group
Training courses can only supply so much. There are some gems of diving technique and knowledge that you will never find on a course. Similarly, some training courses are sufficiently complicated that it doesnt all sink in at the time.
The answer is to get involved and dive with a regular group, whether its a club, dive centre or just a bunch of mates. A lot of knowledge can rub off from other divers on the boat or in the pub.
A regular group also provides somewhere to practise and refresh skills. If training is involved, helping instructors to teach others is a good way to improve your own skills.
Your dive-planning skills can also be brought on by assisting with the organisation of group dive trips. Others in the group provide a source of new dive buddies if your regular buddy is taking a day off.
Each diver brings his or her own experiences to the group and learns from those of others. Its the greatest strength of club-based diving.
On the other hand, the greatest weakness of club-based diving is when members lose touch with the rest of the diving world and become obsessed with their own little empire.
So, having become involved with a regular group of divers, keep an open mind and do the occasional bit of diving outside the group.

Theres time to concentrate on coursework when youre on a liveaboard.
Acquiring rescue skills is a good way for new divers to consolidate their basic training.