Number One: Personal History.
A good kiss starts way before lips ever touch. A reader has to want it to happen just as much as the character does. How do you make a reader want it? Give the reader a reason to root for your character, the history doesn't have to be just kissing history to win over a reader's loyalty.
Number Two: Build-up.
Have the characters spend time together not kissing. Have them not kiss the crap out of each other. Their pasts have been laid out, we now need them to make some history together on the page. They don't even have to be almost-kissing for this build-up to resonate with your readers.
Number Three: Tension.
There are obvious reasons — from page one of the book up until the inevitable kiss — why the characters aren't, in fact, kissing. These need to be good reasons. Conflicts. Maybe they hate each other. Maybe they are off-limits to each other. Maybe the feelings aren't reciprocated. Whatever the conflict, it makes your reader want the kiss that much more.
Number Four: Emotions.
Hopefully, with the use of all of these devices, you've tugged at your readers' emotions, made them feel things, made them long for the payoff. The characters also need to share their emotions. The readers' emotions combined with the characters' emotions is the perfect time for the kiss to happen.
