On Writing Couples and Love Interests

If your characters are a couple out of love then show them in love.
Show them doing things for each other, that they wouldn't do for anyone else.
Show them breaking character because their relationship (or the life of their loved one) depends on it.
Show them expressing their love (in ways other than kissing or sex).
Show them interacting with one another. You know, because couples interact. They don't just exist and then never speak to each other.
Furthermore: I don't care what your couples are into, just make sure they both consent. And make sure that their relationship isn't unhealthy or abusive (and I don't just mean physically.) And please for the love of gods, don't have one of your characters be in control of everything. Members of a couple (or poly-relationship) should all be treated as equals and should all have a say in matters concerning both (like whether or not they have children, or where they live, etc.) This is important because putting abuse, non-con, and unequal relationships in media encourages it in real life (whether you believe it or not) and that's not a good path to go down.
And of course: Your characters don't need to be in a relationship to be happy. Two main characters can come out of a situation as friends, they don't need to be romantically involved by the end of the story.

Also, here are some interesting things to do with your couples.
-Star Crossed Lovers. I just love it when a hero has a thing for the villain or vice versa. Even better is when they both have a thing for each other. Or maybe they were already together to start but the plot has separated them, yet they both long to be back together. Who knows?
-Major cultural barriers between characters. Interracial marriages are cool, both between two different human ethnicities, or between two different fantasy/sci-fi races. Or you could have a completely different cultural barrier. Like a bisexual and a gay or lesbian individual in a relationship together. Or a bisexual with a heterosexual partner. Those count as cultural barriers too.
-More than two people in a healthy poly relationship. Not a harem. A relationship with 3 or more people happily in a healthy relationship together. It's possible and I've seen it happen.
-Fight side by side against adversaries. Seriously, I love it when this happens.