I did the FATs for the Navy at the beginning of July and found that the advice for OASC as suggested above is basically completely correct.
The things I would recommend are:
practising speed, time, distance calculations
being prepared for sitting in front of a computer for about 5 and a half hours with three 5 minute breaks (no sources of advice seemed to mention how long they lasted for)
finally, seemingly standard advice for all military testing procedures - once you have completed a task forget about it - it is almost impossible to gauge how well you have done, so if you think it has gone badly, chances are it hasn't so don;t let it affect the next test.
Oh, and also get ready to be incredibly smug that you have chosen Navy not the RAF when you meet all the people who are at Cranwell for OASC. (only joking all the crab wannabes were lovely)
Finally - Good Luck!!!