Draw them from scratch, and draw them often. Don't be afraid if they start to adjust themselves a little bit at first, that usually means that your art style is probably changing too. Maybe something about their initial design wasn't the best choice (like hair length, or face shape.) just let it adjust if it needs to happen. You'll eventually find them looking the same every time.
Tracing over stuff tends to be more for comedic, or a pause as @Jenny-Toons mentioned earlier. I use it from time to time for that, and it works like a charm. Though sometime's I'll use it too and just use it as a base and do slight changes (like a head angle, or an arm moving) but most the time from scratch is gonna be your best bet. Time and practice will make them stay the same eventually.