Ok, given the amount of, quite honesty, terrible lemons I've come across on my journeys across the interwebs, I'm going to give all you lemon writers a few rules to consider when writing them. Feel free to ignore my advice; I am, after all, an amateur, but, really, even just taking in one of these will improve your lemons considerably.
Rule 1: Use metaphors and Euphemisms carefully
Trust me; some metaphor just make your work really, really unsexy. Be very, very careful when using them. A good way to tell if you've got a good one on your hands is to say it out loud; does it make you feel hot? Or does it just make you laugh? Some of the worst I've seen have been 'her honey pot', 'her triangle of femininity' and anything that sounds anything like 'they soared on the wings of love and exploded into infinity'. They all make you sound like a twat, especially that last one. (Also, try to avoid the word 'member' if you can.)
Rule 2: Explain how it feels, not what's happening
One of the things that turns me off most is getting through the opening part of a lemon to find that the action is described like a piece of engineering. Don't just say 'Body-part-1 entered Body-part-2, making you moan'. Explain what the main character is feeling (both emotionally and physically), not just what they are doing.
Rule 3: The 'Rule of Three' is bullshit
The 'Rule of Three' states that if you include three of the five senses in a sex scene, it will be a lot more believable and generally more hot. That, my friends, is what we call bullshit; you should include four senses at the minimum. It's not that hard people; sight, sound, touch and smell. Include taste too, if it works in your storyline. It'll make your writing way, waaay more interesting.
Rule 4: Hint at a larger storyline
You'll notice that in all the lemons I have ever written and ever will write, there is more to the story then the actual act of fucking. For example; in my Snapped!Fem!France x Reader lemon, the reader is Fem!France's ex girlfriend, who left her for a guy, which emotionally tore Fem!France apart. While I don't show the actual events, they are referenced in the thoughts, feelings and attitude of the reader. Similarly, in my Incubus!America x Male!Reader fic, while nothing actually happens in the writing except the sexual act, the situation in which it takes place is referenced tons because it actually effects the storyline a lot. Which brings me to my next point
Rule 5: Crank up the sexual tension
Make the reader and the character enemies, make somebody have second thoughts, do something- anything!- to raise the stakes, both sexual and emotional. Just describing people having sex is plain old boring; make it into a risk the two (or more!) have to choose whether or not to take. This is one of the undeniable truths in writing; sexual tension will make the description of the sexual act more satisfying for the reader.
Rule 6: Dialogue is hot; use it.
Because having sex in silence would be creepy and you know it. But don't just add random 'Aaaah's and 'Ooooooh's; they are generally mahoosive turn offs. Use dialogue as a way to express the wait for it- sexual tension (see above).
Just a few words can be exciting enough in the right hands to keep a reader utterly engrossed.
I know I may sound like a hypocrite because my 'Roxanne' lemon doesn't have any dialogue in it, but that is for a particular reason; the fic is meant to be creepy- it's showing what happens when love goes wrong.
Rule 7: Don't make the acts the same as every other Lemon out there
Your basic lemon formula is this; the hot guy of your choice confesses to the reader, then starts kissing her, it leads on to him 'pleasuring' her, she 'pleasures' him and then they fuck. I see this in almost every bloody lemon out there.
Now, I'm not saying you can't use any of this, but using all together makes your fic look clichéd and plain boring. Mix them up, cut bits out, add things in and make it as spicy as you like. But please, change something!
Rule 8: Anatomy please!
This is mainly just a side note, but I see a lot of yaoi fanfictions where no lube is used. Now, this will hurt both partners. A lot. Trust me. I'm not saying you have to have everything anatomically correct, but please be logical about big things like this.
Rule 8: To Birth Control or not to Birth Control?
I know that hardly any lemons do include birth control and, quite frankly, I can understand why. But remember, birth control doesn't automatically stop your fic being sexy (think opening condom packets with teeth!) and will stop people being distracted by the lack of it. (Because let's be honest; STI's aren't exactly the sexiest things on earth.) Not saying you have to include it, but you should at least bear it in mind.
Rule 9: Avoid adverbs
This is more of a general writing tip, but try to avoid adverbs as much as you can. Using them creates one of two effects; 'no shit Sherlock' (as in 'he shouted angrily') or 'what the fuck did you just say?' ('She shuddered vehemently'). There are occasions when it's ok to use them but try to use as few as you can.
Rule 10: Show both the physical and emotional side of the story
No matter what situation the lemon is written in, there will be emotions involved. They may vary wildly, but they are always there and they should make up a large part of what the story explores.