I no longer do weddings, but when I did shoot them (in the days of film)...
- I asked for 1/3 to hold the date on my calendar
- I asked for 1/3 by the date of the wedding...and I knew all my costs to that point were covered, for film+processing and preview prints, and my time
- I asked for final 1/3 when I delivered the preview book (which they got to keep).
That way if they never ordered any prints for the wedding album (and it did happen), I did not care. The prints and the album materials were already paid for in full, so it was costs not attained if they failed to order.