Many romance novels today feature love scenes, from the very vague leave-it-to-the-reader's-imagination variety to the heart-stoppingly explicit. It always helps to know ahead of time what publisher/line you're targeting and what their sensuality requirements or prohibitions are. Whatever kind of love scene you write, here are some things you want to avoid:
SENSORY DEPRIVATION: Love scenes, by their very nature, should be a sensual experience, but this isn't limited to touching. Work in as many of the five senses as you can, such as smell or taste. For example, the love scene in my first book, The Maid of Dishonor, opens with a champagne-flavored kiss. Sounds are also good. What does our heroine hear? The hero's ragged breathing? His pounding heart as she lays her head on his chest?
MAGIC TRICKS AND OTHER STRANGE HAPPENINGS: It's easy to get caught up in writing a passionate scene and overlook something. I was once reading a scene where the hero was standing behind the heroine on the balcony, kissing the nape of her neck, holding her close against him, etc...when suddenly he was reveling in the softness of her curvy chest pressed against his own chest. Since I had no idea she'd ever turned around--and in fact still thought he was kissing the back of her neck--this was jarring.
Mentioning that someone had moved would have helped. In other cases, the problem stems from grammar. Terms like "as he..." imply that things are happening simultaneously (he brushed her hair away from her face as he kissed her gently). Using "-ing" verbs can create the same effect. Really bad example to help illustrate a point: Cupping her face gently in his hands to kiss her, he reached down to encircle her waist. Unless you're writing a paranormal in which its perfectly acceptable for the hero to have two sets of arms, this should be rewritten--something along the lines of "He cupped her face, kissing her, then dropped his hands to span her waist." When you proofread, make sure that all the body parts described are lining up correctly and that what you're describing is in fact logically possible (if your heroine is a contortionist, you probably have more leeway).
Also watch for disappearing items. In a very early draft of The Maid of Dishonor, I had the hero standing in the middle of the room, holding a bottle in one hand and a glass in the other (he was about to pour the heroine a drink), when their conversation turned physical. He clasped her shoulders and pulled her to him. One of my critique partners told me she couldn't stop thinking about all that potentially broken glass. Did he smash the champagne flute against the heroine's back by accident? Throw everything down to the carpet in his haste to embrace the heroine? Did the champagne bottle disappear through an interdimensional portal? The last thing you want in a love scene is to jar the reader from the story and have her thinking about something other than the two people on the page and what they're experiencing.
MOOD BREAKERS: There are times in the story where the author will deliberately cut the sexual tension with humor or interrupt the characters in the midst of crucial goings-on.
But there are other times where an author will make an unintentional word choice that dispels the mood--for the readers, anyway. As you select words, consider connotation. For instance, it may be possible to describe the same thing as "moist" or as "slimy." Which one do you think would work better in a sensual scene? I'm exaggerating here for effect, but you might be surprised at some of the descriptions I've read over the years that, while technically accurate, made me wince. Also, be sensitive to the fact that a little bit of passion goes a long way. Unless you're writing a deliberately angry love scene, be careful how many times you use words like "grabbed" or "ripped." Forceful verbs can definitely have their place in love scenes, but make sure the picture you're creating is appropriate to the characters and situation, or your characters may seem to be needlessly assaulting each other. Figurative language can also have an unintended effect. Be careful of any metaphors (especially for male anatomy) that actually make the reader laugh out loud.
While adding internal narrative (i.e. the characters' thoughts) can definitely enhance the scene (see the paragraphs below, on emotion), you want to be careful what they're thinking about and how long they're thinking it. I once proofread what later became a best-selling published book for a friend, and discovered that the heroine had about three pages of thoughts in the middle of the love scene. I got to the end of her mental meanderings and was surprised to learn that they'd actually been making love the entire time. I had no idea that was even going on! While a person can think a number of things very quickly, it looks longer on paper...so the effect was that the heroine was bored and that her mind had wandered for quite awhile. Not quite what my friend was going for!
NEGLECTING THE EMOTION: This advice, as with any, depends on your specific manuscript and story situation. (For instance, you might want an unemotional love scene at the beginning of your book so that you contrast it with another scene later.)
But generally speaking, romance readers want just that...romance. While that may include sex, it certainly isn't limited to physical actions. What sets your book apart, makes your characters memorable, is the emotion. Funny, poignant, passionate, nervous, bewildered...even angry, in some cases. Whatever emotion works best for your story, just make sure you layer it in there.
You've heard that actions have reactions, right? Well, it's the reactions we're looking for here. Say your hero has just caressed the heroine in some manner appropriate to the sensuality level of your book...in addition to any physical sensations she has, show us what she thinks/feels (briefly, not in three pages). In my online story Cara Out of Control, the heroine, because of an unstable childhood, feels the need to always be in control. The love scene in this story isn't so much about what two people are doing in bed as it is about the fact that she trusts the hero enough to finally lose her control with him. And that trust symbolizes their love and moves the story forward. If there's no emotional development in the scene, ask yourself what the scene accomplishes for the story.