Interesting that you chose to post this on the Men's board. Anyway, I think frequent intercourse can "loosen" things up, but only a little. I'm assuming she is well lubricated and she won't complain about that. Doing Kegels will help things stay firm down there but the best thing is what she can do with them. She should "squeeze" you either on the way in or on the way out. She can only keep those muscles clamped down for so long.
Do you wear a condom? Try an ultra-sensitive model.
Use positions that allow her to keep her thighs closer together. Doggie style with your knees outside hers. Missionary: after you enter she closes her thighs and you basically straddle her hips. Lots of stimulation for you and the strong possibility you will be rubbing right on her clitoris with every stroke. Very intense. Lay like spoons with you behind her - she closes her thighs after you enter. The basic idea is to change the angles so that the right parts of you are rubbing against the right parts of her.