This is how to write a romance that won't make your readers roll their eyes, puke, or want to strangle you. (Many of these points are quoted from other users, so it's not all my own material.)
1. A true love = BEST best friend.
The most important rule of romance: you cannot love someone that deeply unless they are your best friend first. You have to be able to laugh with them, relax with them, cry on each other's shoulders, and just all-around enjoy their company.
This doesn't mean you do the cliche i-fell-in-love-with-my-best-friend deal. It means they need to become best friends over the course of the story before developing strong feelings for each other.
2. Love at first sight is a ridiculous and impossible concept.
If you fall for someone the moment you look at them, you're falling for their APPEARANCE. That isn't love. That's lust and infatuation. That's a physical attraction. It's alright if your character feels attracted to their potential love interest at first (though try to avoid this! It's considered cliche and predictable if done incorrectly), but they have to learn that there is more to the person than what they first thought. They could even fall OUT of "love" when they see how terrible the person really is, and then really learn to love them as they share more experiences together.
3. Romance should never be the #1 focus of your novel, even if it is a romance novel.
Sounds crazy, right? Romance novels means it's about romance, doesn't it? Nope. Two people learn to love each other because they shared some kind of deep/touching/horrible/traumatic experience with each other. Let me reiterate that: PEOPLE FALL IN LOVE THROUGH SHARED HARDSHIPS. This is where the conflict in your story comes. No, the conflict is not, "Oh, no! I'm all alone and lonely and no one likes me, but that one hot guy is especially mean to me. I'll just act all snarky/needy toward him until we both fall madly in love with each other for the rest of eternity." If you don't see what's wrong with that, I will slap you with a wet fish.
There should be some REAL conflict happening to drive your plot, and it's through that SHARED EXPERIENCE OF HARDSHIP that your characters fall in love with each other. Again: they need to be friends before they become lovers. Conflict could be anything from here to the moon. The MC could be caught in a war, have lost a friend, killed a friend, hurt someone important to them, was hurt by someone, out for revenge, etc. The main point I'm trying to make here is that romance is NOT a conflict. The conflict of your novel (especially romance novels) should never be anything to do with romance. Romance is a result. Not a goal.
4. Know the difference between "protective" and "abusive".
If you've read Twilight, you'll know what I mean. One partner could of course be abusive in the relationship, but please don't try and portray that as romantic. A guy (or girl) forcing their girlfriend (or boyfriend) to stay home and not see her friends by disabling their car is abuse. Please portray it as such.
5. Please, please, PLEASE, don't describe the love interest as good-looking/handsome/sexy/wonderful/hot or any other synonym of those.
Unless there is a LEGITIMATE reason for them to be good looking that is relevant to the main conflict of the novel (which, remember, is NOT romance), please don't fangirl over him/her within the novel. Do it on your own time, not ours. We don't fall in love with their looks. We fall in love with their personality. Making the love interest really good looking and fawning over them in the narration just turns the romance shallow and superficial. It makes us believe that the MC fell for them just for their looks. This is lust/infatuation, not love.
6. Life is not a Disney movie.
This kind of goes with love at first sight. In most Disney films, the main characters fall in love after one catchy song. Aladdin is how NOT to do it. Mulan IS how to do it. Allow me to explain:
Aladdin: Jasmine originally falls for Aladdin when he pretends to be a prince and takes her on a magic carpet ride. After the "Whole New World" song, they're cuddling and holding hands. Again, if you don't see what's wrong with this, I will slap you with a wet fish.
Mulan: Mulan gets ragged on by Shang, but she's determined to prove that she can be a soldier that he can be proud of. As I mentioned above, the main conflict of Mulan (at the "I'll Make a Man Out of You" song) is that she's trying to hide that she's a woman while training to become a soldier in the Chinese army. It is NOT that she's attracted to Shang or wants to get with him. For most of the movie, there are NO romantic feelings between the two at all, and their love blossoms gradually, through the trauma they face together. Mulan's initial goal is to prove herself to Shang, not to fall in love with him.