Also, make sure that the relationship is about more than the two of them hooking up- they should have more going on than just making out and talking about how much they love each other. There should be a real, fertile friendship beneath that. Unless they're shallow or are together primarily for the sex.
So in other words treat it like a normal relationship between two strangers, that meet and become friends and gradually develops into a mutual caring relationship over the course of many hundreds of pages. As i don't want to create a shallow relationship as i already plan on having a relationship like that between a normal coupling m/f and i don't really want to repeat myself.
Whoa! Whoa! Hey! The last thing we need is a bunch of people going around saying we need less hot lesbian elf action in the movies.