You might want to input a small amount of up elevator before commencing the roll, then down elevator when you're inverted. When you feel ready for it, add rudder, as perttime suggested. For instance, if you roll to the right, you give the opposite (that is, left) rudder for the first quarter of the roll, and then, after 180 degrees, you add right rudder. So, for a roll to the right: first push your sticks in opposite directions, then both sticks to the right.