Who among us doesn't find writing sex monotonous once in awhile? Especially when you've written a lot of it. But the following advice can apply to any kind of scene - action, the climax, etc. Always making sure your scene is unique to your characters is key.
Via Charlene Teglia's blog...
"The thing with writing good sex scenes, as the thing with writing everything else, is this: telling detail, man. Not a chair: this chair. Not a cat: this cat. Not a sex scene, idealized or farcical, but this sex scene, right here, right now, with these two characters having this sex. When you get the off-beat, perfect detail just right, and it makes the scene concrete and real, that’s called fabulous reality. If you have that, you can do no wrong." - Elizabeth Bear
Also, please stop by Word Wenches, the blog I share with my lovely CP, Tara Leigh Coons, and share with us why YOU write. It's amazing how different the answers have been so far, yet they're also so similar.