Jann- Goldie said what I was going to about the 15 mins earlier each day. If your LO is waking at 8:30 and you want a 7:30 wake up, you'd wake them at 8:15 one day, 8:00 the next, 7:45 the next till 7:30 is the natural wake time again.
TBH, I've never had this problem as Nick wakes at 6-6:30 like clockwork.