Buddy sat mine a few years ago and passed the computer based stuff but failed officer selection. Here's the advice I can give you for the computer bit. At home play computer games like a bugger. When you get the make sure you have some change there is a vending machine and you get a break halfway through, you'll need it and you be gasping its bloody hot in the room.
When you get to your station make sure you do anything you need to make yourslef feel comfortable, I took off the tie and undid the top button, took my shoes off and turned my joystick round 180 degrees, I was used to playing games and stuff at home with the opposite to what it was set. Basically mate do whatever you have to make yourself at easy and comfortable.
You are not expected to complete all of them however some are designed to determine your accuracy and others speed.
Keep the dot in the square, using the joystick for vertical and peddals for horizontal motion. It changes form one extreme to the next on a few occasions so watch out.
Next is the beep beep beep annoying one, keep your circle over the others dropping down the screen, its the same path each time so they are looking for an improvement from the first pass to the third. Avoid going away out for a lone one when you have a lot more in the centre.
From what I remember you have number recall they show you a 5 digit number for 5 secs then it disappears and you have to retype it. It then increases up to 6, 7, 8 etc.
They show a row of diagrams showing sequence ie a dot top left, two dots top right, one dot bottom right etc and wan toyou pick from a 4 options what the next one will be in the sequence.
Nest of Cube is in there too, it shows a 3D cub showing 3 sides with symbols and shading on them and you have to identify the net (the flat cross shaped piece of card which would make up the cube if folded).
There is one where you are posed with simple mental arithmetic, memory recall and cancelling out the colours in the background as they pass through the same colured panels all at the same time. Its a bugger.
There is another where a symbol pops up and you have a selection of about 30 below, you have to identify it and type in its co-ordinates on the keypad.
Also is the identifying 3-D aircraft from the instuments showing pitch, artifical horizon, compas etc. (everyone in the room will be using their hands to work this one out)
There are more but there isn't much you can do for any of these except pay games on the computer and practice memeory techniques. Also keeping stuff in your short term memory, ie turn a pack of cards over and arrange the deck into 6 rows, turn two cards over and turn them back until you get a pair. thats pretty good.
But don't get worked up and don't go down stairs and tell eveyone how you've mucked up, they are in the same boat and it gets boring.