I can't even break 500 on a practice test! =(

I think 420 - 480 are typical for a diagnostic test. so first 2 or three tests are usually in this range. So lot of hard work needs to be done to get into higher range.

First of all my question is have you covered the whole subject areas as far as studying. ??
Whats your background are you a math person? or a verbal person?

In math: permutations and combinations, probability, sets, numbers, strategic ways of working on problems like fractions? ratios, etc.

In verbal: Have you got enough practice on critical reasoning? How about sentence correction? Have you got enough practice on reading comprehension?

Work on each of the areas as if you are working on your muscles. For instance hamstrings at one time. biceps at some other time. triceps at some other time.

Make sure that you focus on each of the areas. Whats critical is you should be able to finish the problem in 5 to 10 minutes when are not timed. Timed working on problems is differnt from untimed. try to work on hard questions by taking as much time as posssible.