At the lower weights, it was strict behind the neck overhead press to get things moving prior to doing the overhead squats. Admittedly, overhead squats are especially comfortable for me, but it's good to make sure I can maintain good overhead position for a while, especially if I need to hang out at the bottom of a snatch for a bit to stabilize before standing back up.
At heavier weights, I used behind the neck push-presses to get the bar overhead. Realistically in setting up for overhead squats, push-presses or push-jerks are the preferred movement to get the bar overhead, especially at heavier weights and/or higher repetitions.
And then there were the clean & jerk drills, which focused more on maintaining on balance, position, and dropping under the bar more quickly. I find it much easier to drop under the bar on the clean than I do with snatches.