The problem with giving general tips on this sort of relationship is that it really has to be approached with the specific characters in mind. How much do they hate each other, and why? Are they both hedonists? Is it a case of corrupter villain and/or self-destructive protagonist? Do they have a history that might create an emotional connection (or habitual physical attraction) despite them hating each other now? Do you want them to actually hook up (as a fling or on a more long-lasting basis), or are you looking to keep things subtextual?

tl;dr, you'd be better off evaluating the characters and their individual behavior and motivation patterns, then plotting your course, rather than trying a more general approach.