ahh... I agree..... It's difficult to memorize .
I have TOEFL/SAT/GRE/GMAT application made by Kaplan on Iphone. It's not bad . and I found a series of GRE-vocab on Smart Fm.
The former is mobile , so sometimes I check them in my spare time for example , in restaurant and coffee store but The latter is web-based-application. It's fitting in learning at desk . Good luck !