First, a disclaimer: I am not the be-all end-all authority about writing love scenes. That being said, I've written and edited a lot of them. So I've seen a lot.
One thing I've seen too much, especially from newer writers, is what I call (not uniquely) the "insert part B into slot A phenomenon". The author dutifully describes the act of having sex. He does this, she does that, this goes here and that happens. The words are different than that, of course, but the reader has the same reaction in every case: Zzzzzzzzzzzz.
Zzzzzz is not good. Your reader starts to lose interest. They begin to skim. They put the book down. They wander away. You've lost them. Not what you intended to do when you began writing the scene, or the story. Why? Because you've failed to make an emotional connection with your reader.
Your primary goal as a writer is to make an emotional connection with the reader.
An emotional connection is the key. That's what keeps them involved in your story, keeps them reading--and makes them want to buy your next book. (Okay...so maybe that's your primary goal, but it sounds so--ugh--materialistic. We write for writing's sake, right? Of course.)
Well, that's fine, you say. But how?
Get in touch with your character and your reader will be involved.
Add some emotion and physical reactions, but avoid using adjectives and adverbs. Oops. This is the kind of directive that will have you running to your writer buddies saying , "WTF?" Okay…let me see if I can explain it better…say your hero caresses the heroine's soft breast. That's only a surface description. (Insert hand A over part B.) You have to go deeper. What does his touch DO to her? Does her breast heat/tingle/insert reaction of your choice here? What does his touch do to her EMOTIONS? Does she love it? Hate it?
Of course, you don't have to go deep into your character's psyche and experience with every caress, but you have to build some layers.
Layer that scene like a lasagna!
Every love scene needs layers. You want to think of the Body/the Head/the Heart. (Physical/Mental/Emotional). Otherwise, you'll end up with a flat scene that reads like an instruction manual. And that's what I see, too often. The new writer stays at the surface of the action without adding those crucial layers that make a reader a part of it. Because by giving your reader those pieces, she (or he) will be able to put themselves in your characters' places. The connection is made.
Now, get to work. :)