Pretty basic stuff but you have to be sharp. It's not the mental maths that will catch you out, it's having to seek out the correct information you need for the SDT calculations under significant time pressure. Plus when you have 2-3 other tasks to do simultaneously whilst working out sums.
What I did was jump on flight sim, select a mission with a high workload environment (I used the redbull time trial course, high-altitude and blind instrument approach missions) whilst having someone ask you SDT questions and general multiplication, division and subtraction questions.
My thinking was that this sort of preparation has multiple benefits when it comes to the CBATs:
1) Develops your physcomotor skills if you use a joystick
2) Familiarizes you with a cockpit environment, i.e. basic instruments (if you aren't already familiar)
3) Develops your ability to multitask whilst performing mental calculations
Apart from getting SDT squared away and doing the above, there isn't all that much that you can practice for. Maybe do some verbal reasoning tests for good measure.
Hope that helps a little!