If I'm in the right headspace, servicing boots and feet can really get me going. I actually had to be taken out of chastity in a recent scene because servicing his feet while he was abusing my balls had me hard for so long my balls turned a really dangerous-looking shade of purple. I've got a few guys who, just to frustrate me, won't let me lick their boots/feet without permission, even if they're in my face. Being gagged when either is in my face usually gets me frustrated enough to start whimpering pretty quickly. Reversing roles I've had some success frustrating the HELL out of guys who like kissing or sucking, or really any such fixation.