I noticed a couple of people asked about the UKcat so I thought I'd post my advice for it (did it 2 weeks ago, average 777.5). Some of it might be a bit late, but I'll include it anyway.
Preparation
I used the Kaplan book and Medify. I bought the 600Q book too, but much preferred Kaplan. It's got excellent advice about how to approach each section. I read their advice and then worked through the questions in the time frame given. I had two weeks of Medify and would just have a play with it in between doing other things - basically, if I was at the computer and had five minutes free, I'd do some questions.
What I practised most on Medify was my pacing and scanning for key words. Pacing is fairly self-explanatory. I still ran out of time in VR (my weakest section, 670), but I found the exam to be a lot more comprehension that true/false/can't tell questions. I had to kick myself into moving on to the next question if I couldn't find the answer. Be ruthless with this: pick a choice, flag, move on. Know your timings. The final 3 sets of questions might be easy but if you've spent half the time on the first few, you're not going to get a chance to get to them. Same goes for QR. if I felt that the maths were going to take me too long, I flagged and moved on. I got 890 in this, so clearly, this worked for me. By the end, I had plenty of time to get through the questions I'd flagged.
AR baffles me. I got 830 on it, but I can't tell you how. I saw about 40% of the patterns in the exam and went with my gut feeling on the rest. Again, this was a timing thing. It's about a minute for each set, so I'd spend 40 seconds looking at the pattern, if I found it great, not guess. The same thing as above applies - don't miss the 20 questions because you spend your time on 5.
DA my approach was the basic. Write down the code and then, if none of the options jumped to me, exclude. One option might have the plural in the wrong place, one might miss something. Eliminate as much as possible and then choose. In practice, I got about 90% on medify, so I was expecting to be better than 720, but them's the breaks.
If you get Medifiy (and I can't recommend it enough), save the mock tests for the days before the exam. Four days before, at about the same time you've got the exam, sit down and do them with as few distractions as possible. This isn't always possible (work and stuff), but try to get your brain used to having to do a lot of work in one go. I've been out of uni for a while, so it was good practice.
Exam Day
Double check the pens and keep your writing as tidy as possible. Getting a new notepad is time you don't get back.
Take the whole minute between sections. You should know them by the time you get to the exam, so spend it stretching and relaxing. I thought of songs to clear my head. I was convinced I'd botched AR and knew I didn't do marvellously on VR (or not as well as I wanted to anyway), and I psych myself out easily. But every section is different, and if 3 play to your strengths, you can balance out a mediocre fourth.
Eat food and drink some water, but nothing too close to the exam. Again, bathroom breaks cost time.
None of that is very original, but it's a summary of my experience with it, anyway. I'll answer any questions you've got about it or the Gamsat (sat it last year, got 68 overall - 69,66,69).