When it comes to online tests the best way to prep is practice. Make sure to prepare in advance studying the most common test (math, logical, verbal and situational).
How to practice?
There are several banks you should apply to and each bank has a different provider for online tests. The best way is to apply every year to the same banks and take notes of the online tests. Generally banks tend not to change the questions from one year to the other.
We also recommend you take the test with some friends so that you have much better chances to choose the right option.