Well, now my husband is going to know my secret buuuuut... what I do is go down on him and while my butt is facing the other way and he's very focused on what my mouth is doing to him, I let'em go. This only works if you're pretty sure it's going to be silent. Also it helps if you have gassy dogs, so then if he smells it you can just mutter "damn dog" and move on.