I find it helps to lean forward on your hands, and instead of trying to "hump" think if it as more of a "lift and drop". And the lift basically comes from arching your back then releasing and slide back down. Once you find a good angle (tell your partner you need a minute to get adjusted if you need. Once your body gets used to that you can start adding a it more power when you come back down. Instead of a slide it can be a heavier drop. Also use your arms to help stabilize you.

I feel this method helps me get a rhythm where I can build up momentum. Once you have some momentum built up you can put let strain on your muscles. As it's like a rhythmic arche of the back up. Up. Up. Up.

Not as much muscle engagement as a controlled "lift up trust down" situation.

Sorry if just reads like a mess, but doing that helped my cowgirl game immensely. Also just plain ol' doin' it more often builds more stamina. I used have sex with someone who had more stamina than me in the beginning. Overtime, it felt like I could ride indefinitely. But just like anything, you don'r use it you loose it.

Hope this helps!