First of all, practice. Secondly, practice. Third... guess what? Practice.
It really is that simple. HOWEVER, there are tricks to use to make that practice more valuable. I did the UKCAT for the first time in 2010, and achieved a score of 665. At this point, I didn't fully believe that you could 'revise' for the UKCAT, and so I geniunely thought that was the best I could do. In 2011, I did a fair amount of work (not a lot, mind you) and improved my score up to 752 average.
So let's just go over what I did differently, and hopefully that will give you some idea of how to do your UKCAT right the first time around.
1. Do lots of practice questions - I had a book with 500 of them in, and it was invaluable.
2. When marking your practice questions, consider where you went wrong. Did you make a typo in the calculator? Did you misread the text? Have you completely forgotten the difference between mean, median and mode? The different types of questions in the test are what make it complex to revise - so consider where your own strengths and weaknesses are to know where you need to improve.
3. This one is specifically for abstract reasoning, as that was my weakest area: Have a system. Have an order that YOU do things, every time. This is where I can finally see the relevance to clinical practice.
For example, when interpreting ABG (arterial blood gas) results, I start by looking at the patient details and any relevant clinical details (aka, why was it done?). Next, I'll look at the pH, then I'll consider what the acid-base disorder is by looking at the CO2 and HCO3-, then check for any compensation. I'll then look at the O2 level, and finally finish off by looking around for any other abnormalities and seeing why this might be relevant.
My point is that I will ALWAYS do the same thing. No matter what. I'm not going to miss anything by doing it that way. So count the number of shapes, check the colours, count the sides, count the angles, check the positions... whatever works best for you. The only way to make sure you won't forget something is to have your own system, and stick to it.
Finally, if it doesn't go to plan, never mind. Your UKCAT is only part of your application, and you can easily minimise the impact of a bad score (as well as maximising the impact of a fantastic score)