Yuri from the Gymnastic Bodies Forum made a video on progressions and assistance exercises to learn the Press to Handstand.
- seated straddle leg lifts for compression strength
- from a lying hollow position do the movement , try to keep your lower back on the floor, finish with active compression
- box presses
- isometric holds in weakest position
- slow negative (against a wall)
He also added the following:
Belly to the wall handstand, lean shoulders forward a bit and come back so you are working on forcefully opening the shoulder angle from a slightly planched, closed position
Helps a lot with more than just presses
For more information check out:
- Press to Handstand Resources
- Feedback I got on my Press To Handstand (lots of good information here)
Update: Here is another good video with Progressions courtesy of CrossFit Gymnastics coach Sean Lind.