I could write a book on it if you'd let me u.u . I think the biggest things to keep in mind is to not make them perfect. Give them some flaws, or else they'll be boring and there'll be no room for growth. Funnily enough, I see my flawed characters getting the most love. People /really/ love Pat and he's got the most glaring flaws of the bunch.

I think it's also a good idea to construct a romance route from a plot development perspective. What challenges will the couple have to endure? What kinds of things are they going to learn? How will those lessons strengthen their relationship?