Competition painting boils down to a simple formula: Practise, Practise & Practise.
Take look a Cmon's gallery at the highest scorers this is the minimum level to be looking at to 'Knock em dead' at a competition.
But for the Figure itself
1) Preparation, make sure ALL molding lines are removed, even if they only show up once you've primed the mini.
If the cast has imperfections in any area, Fix them. Milliput paste is a great way to smooth air bubbles in the metal.
2) Colour scheme, plan your colours in advance make sure that they work together.
3) Take your time to paint. If it means you only have got the face done in a painting session, fine. Doesn't matter, take the same care over every area.
4) Focus. As was very nicely pointed out to me last weekend(*) you need to get a point of focus on a mini, mostly this is the face.
Get that right and "Oooh Boy" you're well on the way.
5) Lighting. Everything has a highlight and shadow. For a miniature these are easily defined in the sculpture, but don't think that every raised area has to be highlighted at the same level. It's an accepted constant that most figures are painted as if the light is striking the mini at roughly 45 degrees from the right. (when looking at the mini).
This means that the lower and left hand side should look slightly (not much) darker than the lit area.
6) Contrast. People forget that they are looking at something which is roughly 1/56 th the size of a human so you need to force the contrast up in order to compensate for the scale. Sometime that means making a decision to paint a highlight crease almost to white even when you think you shouldn't.
7) Basing. Painting competitions aren't about gaming with your minis so a standard gaming flock base isn't going to cut the mustard against a good story telling scene. Paint the base with the same level of attention as the miniature. Learn to make the most of how your mini looks, if its a static pose then a base with lots of fine detail might not go amiss. However a dynamic action pose will work better with less clutter.
One thing you must remember, Make sure you mini can be seen. No good painting a mini to almost reality levels if you bury it behind too much foiliage, also (another good tip I recieved) always make sure that the base/groundwork darkens slighly away from the mini (as if he or she is standing in a pool of light) this helps focus the attention on the mini.
8) Failure. To quote Adam Savage "Failure is always an option".
You aren't going to win every competition, some people are going to "kick your ass" in painting competitions. Good get used to it, no matter how disappointing it is, consider it the challenge to raise the bar on your own work and get to the point where you can make others consider you a 'threat'.
Always remember however that its people you are up against, when you win and one day you will win something, be nice about it.
(*Many thanks to Mike and Ali McVey for some Simple and Very Helpful insights. Much Appreciated. And when you get to Bolton the coffee is definitley on me.)