If youre on the newer versions, there's now the side position to consider. Thus:
undress as needed
Shift hands to be raised
Shift body to side position (click on her side, slightly above her belly and below her chest)
Find the "Shift body" command in the side position (with arm tucked in it's usually a bit lower, slightly above her rear. Otherwise, with arm shifted, it should be along her side slightly above her hip near her belly)
Congratulations! She should now be on her belly. You can proceed to headpat her and shift her legs to access her ass, assuming that you've undressed her enough.