Try what guys refer to as a full house. Snap the ailerons and the rudder in the same direction at the same time. My Sturdy Birdy II rolls quickly. When I use just ailerons, it requires LOTS of down elevator in the inverted part of the roll. Using the rudder and ail, doesn't require much. Also, bring the nose up just a little before you roll, I've found this to help ( a guy at my club suggested it ).

I've also found inverted flight to be easy on a trainer. Remember, the ailerons work the same whether the plane is inverted or not...

Give yourself altitude first, then start a roll with ailerons and don't be afraid to push the elvevator down!, then fly upside down for minutes at a time. It's fun and sharpens skills.

-Dan