Don't forget that sex, by definition, is a sensual act and that we all have at least five senses...six depending on how metaphysical you want to get. Don't do them all each time but in order to approximate reality, at some point you will have to describe not just the physical and the visual but the sounds, smells and tastes of what your characters are going through also. Remember, too, that the biggest human sex organ is the brain. Much of the sexual tensions between your characters can and probably should come through when they are not being physically intimate in your story.
Dialogue. Don't just describe what's happening. Have your people talk. When you write dialogue, don't worry about being grammatically correct. Write the way people speak. Read it aloud and if it sounds wrong, change it. If you can't hear it yourself, have a friend read it aloud to you. Where it makes you cringe, fix it.
If you write in third person, stick with that throughout. The third person is the omniscient narrator who usually shows no emotional involvement but can see all of the characters at all times and describe their emotions. If you choose to write in first person, you open up the possibilities of characterization for your narrator but limit yourself to only his or her point of view. Another important thing is tense. Most stories are told in past tense. If you choose to do this, then every single aspect of it needs to continue to be in past tense. Present tense is grating for some but if you choose it, again, make sure to be consistent.
Talk to people. If you're a guy, talk to male friends beyond all the bragging and listen for the actual emotions that come through in everything from long married sex to no-strings attached one night stands. Talk to women friends, too. If they don't perceive you as threatening, you will be absolutely amazed at just how much they're willing to share with you. Use all of this information to make your characters come across as real. Even the most perverted fantasies become more arousing when they are grounded in reality.
Most of all, have fun! Writing erotica is a liberating exercise in creativity. It's hard to hold yourself back and expect any success. Avoid cliches, emphasize character, use proper grammar and realistic dialogue and you, too, can become a successful international pornographer!