The most critical lesson in writing a love scene is that it is similar to making love in the real world: when done well, it is messy, chaotic and somewhat animalistic. The civilized approach does not work; it leads to the greatest drawback of all: predictability.
Does this sound familiar? The leading man and woman dislike one another intensely; something happens and they see another side of the other; in spite of their best efforts to deny it, they find themselves attracted to one another; and they ultimately fall into a passionate embrace. Do you really want to write that one again?
The Best Love Scenes
The best love scenes are the ones in which the participants are not perfect specimens and the circumstances are obscure and somewhat confused. They are the scenes in which the reader has to work for it. In a word, they are authentic.
For example, here is the opening paragraph of my book, Point and Shoot:
This is how you make love to a woman undergoing cancer treatments. You ignore the metallic taste of her kiss; the slight snorting sound she makes when you press into her; the bony feel of her body, covered by skin that lacks tensile strength;...