I've taken the theory test twice. Once in 2008 and then again in 2012...had to take it again because test certificate expired in 2010. The first time I got 49/50 multiple choice and 54/75 hazard perception....second time I got 48/50 multiple choice and 61/75 hazard perception. Both times I used the official dsa theory test cd, its really good as you get practice hazard perception clips and mock multiple choice tests. The hazard perception is the bit that causes some people to fail...my advice would be do not excessively click, if you do they will give you zero marks for the clip. Also when you do see a hazard (pedestrian,tractor, horse, cyclists, cars approaching from side roads, cars pulling away etc) I would click as soon as I saw it and then again just in case. Also if you see a cyclist I would click and then click again if the cyclist swerve, or I would click if I saw a pedestrian and then again if the pedestrian starts to cross the road. Just give it your full concentration , click when you see something that you might have to slow or stop for, and its fine.
For both tests I revised for about 2-3 weeks in my spare time. I then wrote down any questions I was unsure of or kept getting wrong, made sure I went over them until I knew them 100%, I also was revising all the way upto the second I arrived at the test center