#verb conjugations

I'm having a bit of a crisis with verb conjugations in Spanish. Specifically, the subjunctive mood is killing me. I know it's used to express doubt or uncertainty, but when to use it is just not sticking in my head. I've been trying to practice with some online exercises, but I still feel like I'm guessing most of the time. Anyone else have a hard time with this? How did you finally get a handle on it?