Dru,
I went to flight screening about 5 months ago and my recommendation would be to do about 5-12 hours of flying with the majority of those hours being in a glider. I recommend that you start out gliding and then around 3-4 hours in an aerobatic plane.
Gliding helped me out ALOT at flight screening as you get a good feel for rudder and aileron co-ordination. Definately get some hands on flying under your belt before you go, however a few guys on my flight screening course had 0 hours and were recommended.
Hope that helps and good luck,