Before we talk about writing romance, a word from Bret and Jermaine to get you in the mood...
Now that you're all ready, (and hopefully wearing nothing but socks) there are some important things you need to know before you start writing romance, or including a romantic subplot into your novel. Which I highly recommend at least having a romantic slant to your fiction because let's face it people, romance sells as I pointed out in this blog post here.
The number one best ingredient/tip/nonsense I can give you about creating tension of the romantic kind between two characters is this: they have to be aware of each other.
Not just, "Oh, yeah, there's that dude and he's hot" awareness. No, no, no, my friends. Your love interests need to be hyper aware of each other.
I touched on how important it is to have a self-aware character here. But when you're making busy with the romance, your character needs to be more aware of their soon-to-be-significant-other than of any other character.
Even if they don't like them at first. And they need to let the reader in on what they're thinking or realizing about that person as they get it. We (the readers) need to watch them fall in love. So you (the writer) need to make each scene between them count.
Keeping that in mind, the quickest way to make the sexual tension disappear (like a bottle of wine in my house) is to have the characters admit their feelings too early in the story. They need to be unsure of each other and play off of each other's uncertainty to keep the reader on edge and turning pages.
Sexual tension = unsatisfied attraction.
Make sure there's a good reason they can't get together right away, and at the same time put them in a situation where they are pretty much forced to be around each other a lot (i.e. they each need something that the other has or can provide).
It's not easy, but I know you can do it!
Now get down to business.
(and good luck!)