If all fails and time's running out, just lay back and tell them how much you want them to cum all over you. It works 99% of the time for me.... they're so used to their own hand, that it's very likely it'll work, and you're the 'porn' to which they're touching themselves, if you get me. Don't get me wrong I always prefer to get my clients there myself but sometimes this is the best solution when you've tried everything else.