In my opinion, practice tests are primarily useful for two things --
1) getting a sense of where you are at in terms of your score
2) figuring out your weak areas
Repeatedly taking practice tests is not the ideal way, however, to study, or improve your score.
In order to do that, what is best is to study one problem type at a time, in-depth. Become automatic at the easy versions of that problem type, and get to the point where you can anticipate the nuances of the hardest versions of that particular problem type.