The OG is closer to the real thing than Kaplan is. Also make sure to do the PowerPrep and GMATPrep CATs.
But a good study regimen has very little to do with what books you buy, and a lot more to do with how you use them. Instead of just going through questions to get them done, take your time, go through them slowly and spend time writing out explanations in longhand as to why you got to each answer. This forum is a good place to do that. Make sure you really understand all the concepts inside out and backwards. Then work on your timing with a stopwatch to pace yourself.
Good luck! All the stories on this forum of people who pulled up their scores by 100+ points should hopefully be inspiring to you.