Because anyone worth doing is worth doing right, we asked the owner of new-school sweat-factory PUSH Gym (himself a personal trainer and MS in physical therapy) to develop a curriculum of exercises specifically designed to improve your form, endurance, and flexibility in the bedroom, though getting someone there via a successful bar pulldown is still up to you.
Back Bend: Flexibility = more comfort/length in all positions, so start with a medicine ball stretch. Lie with your back on the ball, and with your arms over your head, lift your body up off it -- as an added bonus, this warm-up will gradually increase back flexibility, letting you inch ever closer to fully s'ing your own d (...stretching your own dorsal, of course).
TRX Strap-Squat Thrust Press: The real action begins with this single motion designed to up your thrust power. Grabbing suspended cables, perform a deep squat, and then pull yourself up by "thrusting your hips up in an explosive manner", marking the one explosion your girl will be happy to see happen early in your session.
Dumbbell/Kettle Bell Squat/Curl: This combo movement'll harden the specific muscles needed to lift and hold her in the long-admired standing configuration, no longer just for dudes who live in the San Fernando Valley. Clutching free weights at your side, perform a squat and bicep curl at the same time, then push them up over your head.
Yoga Junk: Start in a downward dog position, then “float down into a pushup”, curl your back into an upward dog, and then finally push back into a downward one again, all of which will advance both the hip/back dexterity necessary for a robust-enough missionary to make her a believer.
Just ask for Marty at PUSH over the phone, and he'll even hook you up with a free workout to show you how all of these movements are done by a real pro, recalling that other old idiom to live by: try to schedule your sex-thrusting, personal training sessions during off-peak hours.