This is not a tip or trick but comes from hard earned experience.  I have made all the mistakes for you.

If I wish to do well, I must not use sugar nor refined carbohydrates of any kind. I must not use caffeine products. All of these products do something to my brain that makes me unable to think straight.

I have also learned not to eat or drink anything except water at least one hour before studies and especially before a test.

If you are able to master the Khan Academy tests doing these things, I applaud you.  If your are finding the going difficult, try my method. It just may work for you. 

May the force be with you.