Practice stepping and footwork drills as often as possible. When you do it, practice stomach breathing. What I like to do is go for a walk or run. Then when I decide to turn around and go back home. I practice my footwork drills all the way back. I'll take breaks when I have to. Without good stance, footwork, coordination between upper and lower body, and proper breathing, you'll never have any decent power in Wing Chun. In addition to that, practice the deadlift exercise. You'll strengthen every muscle in your body as well as your endurance. Do reps between 15 and 20 and take several deep breaths between reps towards the end of your set when you get tired.