there's only so many ways to stretch the legs. Unless you want to starty an overall flexibility program like Yoga, you're going to be kind of limited, but search for "EXRX" and use the "exercise body map" for other stretch ideas. It sounds like you are already doing the standard quad and ham stretches. It is better to stretch after you exercise because your muscles, tendons, and ligaments are warm, and as a result, more pliable making stretching easier. I don't think it really prevents soreness. You could strengthen legs with weights, but I'd recommend sprints if you want to increase running speed. If you can do drag springs (running while dragging something heavy) it will help. Otherwise, I'd go with plyometrics or squats and deadlifts. Stronger legs will help you run faster. Look at the thighs on Olympic sprinters.