I think the secret really is finding the right position. To me it is a very submissive thing and honestly it shouldn't be too much work for her. I find that on a bed on my knees works best assuming the bed is not too high. I pull my knees under myself to lower my ass to the right position for her. Once you are in a spot where ass and strap on are at the same level you should be pushing into her thrusts so she doesn't have to work so hard at it. After a little while, assuming she really enjoys it, she's really so into it she's forgotten it's work.
Another thing you can try with strap on play is combine it with rope play. It doesn't have to be as complex as the picture below, but if you can get some rope to grab. The sub can be quickly moved to where ever you need him so it's easier on you. You can even guide his motion to let you just stand there an enjoy the scene...