I used to have this problem too. You just gotta have your body properly aligned (parallel) and stacked over your board. Don't do unnecessary movements, one technique my friend thought me is imagine that you're an arrow and just point downhill. If you do that you will be able to flat base and go straight. Another thing to picture is that if you just let a board go downhill it will go straight and will not catch an edge. Make sure to keep your knees and legs loose so you can absorb bumps.