From direct experience, if you tie her in a standard rope body harness, with crotch rope, one that will slowly tighten if you grasp it elsewhere, you will soon have her fucking the rope.

If you restrain her arms to boot, she will have no relief but to beg you for her orgasm.

Personally, I like double coin knots up the front, split square knots on the back, and a nice fat square knot on the clit. Later, when she relents, you can relocate that knot a bit higher and use the opening to insert and secure some sort of nasty vibrator. A dragonfly armbinding will not only restrain her arms, but combined with the body harness, will leave her somewhat breathless.

Check out two knotty boys (tkb dot com) for video downloads of the knots and bindings.