My girlfriend often gets tired during them. Although I can tell she's enjoying herself, I think she literally works too hard. I've tested this out solo, and I've found if you tense up your arm muscles like you want a result asap and go as fast as possible, you get tired at an incredible rate. I think that's what her problem is. When I solo, I just kinda "throw" my hand up and retract it down, and it just kind of repeats itself with minimal effort.
I told her that she tries too hard, and she just smiles and says "Well you weren't complaining, were you?" No, I certainly wasn't!! But sometimes I feel kind of bad because she'll lay back afterwards and just go: WOW!
So anyway, ladies, what do you do to prevent yourself from getting tired so fast? Is there any special technique you use? Do you just go insanely slow until he's about to blow, then cut loose? Just trying to find a suggestion for my girlfriend to help her out.