Form: When from kneeling, make sure you head goes to the opposite side of the knee that is up. Put your hands in a praying-type position on the same side of the knee that is up. Roll backwards across your shoulder blades. You neck should never touch the mat. Use the leg that was down to kick/thrust a 45 degree angle in the air and your hands to push on the mat for the follow through. It goes the same from standing.
As far as the rest goes, it sounds like you have a momentum problem. Start the roll with more energy and the leg thrust and hand will help with this as well.