This tutorial covers the most important points when starting to train for the planche.
- L-Sit to tuck support
- rock back and forth between tuck support and tuck planche
- play with various leg positions , one legged , straddled, both legs kicked out
- correct hollow body position
- locked out (straigh) arms
- tucked planche push-ups
- correct planche lean (also see here), gradually raising feet
- hand placement
Also Check out this tutorial which goes into a bit more details.
Update: Here is another Planche tutorial
this one also shows lowering from handstand to straddle planche
Update #2: various auxiliary exercises using an exercise ball.