I think there is a big difference to wanting a man to come and leave in the first 15 minutes of an hour booking and trying to get him to hurry up towards the end, especially if he's annoyed and mauled at you for nearly an hour. I can't be the only one who wills some of them to come just so he'll stop speaking/leave! Am I the only one who has yanked away at a cock in revenge for his rough treatment of my clit?!
Anyway, I think we can do our best helping a man to come and it's sometimes worth asking What will do it for you because sometimes they can be helpful by asking you to talk dirty, play with yourself, call them daddy, shove a finger up their arse or whatever, but I think it's tricky to do these things unless you know them because you don't want to horrify them and put them off completely. Essentially they have to take a bit of responsibility, they know themselves and what turns them on so they need to help us by either accepting they won't come in X amount of time or need something more kinkier to get off.
I also think a lot of men have what I call Wank Hand in that they have spent so much time wanking their penis is programmed only to respond to their particular grip and just can't come any other way. If things aren't working I will sometimes say why don't you wank over me/come on my tits and let them do it themselves. The combo of them doing it their way and getting to come on me often works a treat!