Well, I'll admit that I only recently started writing WAFFy stuff. But here's what I typically do:
1) Become each of the characters. Don't be afraid to step into their shoes. Hack into their minds. Ask yourself what would they say/do in this situation.
2) I actually base some of my Lois and Clark interactions on how my husband and I interact. Then I tailor it to make sure that the actions/dialogue fit the characters of Lois and Clark.
3) Don't shy away from comedy. One of the best things about Lois and Clark's relationship is that it grows from a best friendship. So they are used to the good natured teasing that goes along with the territory. They aren't afraid to tease on another, sometimes even in slightly ridiculous/hilarious ways. WAFFy stories without humor can get a little dry and heavy I think. Humor keeps it light and refreshing. Don't make it a ROTFLMAO-fest but don't allow the story to take itself too seriously either (unless of course, the story demands going to one extreme or another).
4) Allow your inner 5 year old to play. This kind of ties into #1. When in doubt, act out the scene from both characters perspective. I don't mean that you should make out with your pillow pretending that it's Clark, but sometimes saying the dialogue aloud in the proper tones can help you decide if a scene is working.
5) It really can be all about the details. You can write humourous dialogue between the characters, but the little touches like Lois brushing the lock of hair from Clark's forehead can add that extra little moment of "awwww."
Anyway, just my two cents worth, even though I really haven't written much in terms of WAFFy stories.
(Snowbound was the only story that I wrote with the intention of writing a WAFFy story. Any others with WAFFy scenes just sort of emerged in the writing process.)
