I didn't take a prep class, it was all self-study, the trick that worked for me was to just put in the hours. I banked nearly 150 hours over the course of June and July, it was tough to do because I was also working and I very nearly burned out. I also read everything I could find on this forum. It was a good way to stay motivated, seeing what other people were doing, and struggling through the same thing.

For sentence correction, manhattan GMAT guide

For data sufficiency, just do a lot of them, you'll get better

For RC and CR, I'm a grad student so I had preparation reading complex passages

For problem solving, there are only so many types of these questions, learn them all and you're on your way.