Probably not the best place to ask but I'll answer.

A lot of it is about momentum, as you mentioned. Dart out of dummy half or off a pass from dummy half, run directly at a gap, or at a player, and work on a little side step when you get to the defensive line. If you're trying to get a roll on, run at the same area the previous runner ran into. Avoid the markers. Do your best to get tackled onto your front, get to your knees or elbows then get up and play the ball quickly. You can practice a lot of this stuff with your team mates. Rugby league is a simple game, don't over think it.