The potential test's main difficulty is time-related: you need to learn to go fast yet still be accurate. There aren't many practice exams available on the web yet, but a solid '2nd best option' is to practice the PST exam. Those you will find in droves. PrepLounge has some excellent material, which will take you a long way already.
Don't forget also to prepare the quick math calculations; caseinterviewmath.com in particular will rank you by accuracy and speed.
Check PL's material there:
Hope this helps -