I think mental imaging is helpful. I try to think of myself almost as a string, keeping the body straight and long. Keeping the face down with it looking at the bottom of the pool (if you are in one) is helpful. This tends to keep your butt and legs up a bit, which I think can be the hardest thing to do. Making sure that your elbows are relatively high as they come out of the water is helpful, too. And, of course, making sure that you have a suit that is not compromising your ability to be streamlined is important, too.