Never refer to anyone's tongue as their 'wet muscle', because that just sounds gross.
Medical terms for parts of the body are forbidden during sex scenes. Really, it just kills things a whole lot.
Nobody ever 'just knows' what someone else is thinking, unless they're twins, and even then it's not all the time.
Eyes are the windows to the soul. That does not, however, mean that one glance at them can tell you the entirety of what any given person is thinking and/or feeling.
Don't ask obvious questions in your summary. "Will they recognise one another?" "Will they be able to fall in love again?" Well of course they will, you'd have no fic if they didn't.
There are a bunch of obvious, yet decently elegant euphemisms for a guy coming that you could take your pick from. Don't ever use the word 'squirt'.
Unless your character is deaf, he/she can hear. Unless your character is blind, he/she can see. Unless your character is Jin stupid, he/she can think. Unless your character is Ryo a robot, he/she can experience emotion. Description is the key.
It is accepted canon that Ryo has a quick temper. This does not make him six years old, and as such he is unlikely to throw a tantrum and storm off every time he is disagreed with.
Opposites attract. However, two people who each possess every quality the other dislikes in a person are not exactly the likeliest of candidates to fall in love within a week and immediately start to call one another 'darling'.
It is very, very possible to get angry at someone you're in love with. In fact, fighting every so often makes for a healthier relationship.
Not everyone is an excellent kisser, or really good in bed. Even if you're head over heels for someone, they might be a crap lay.
If someone's proud, they're not about to go ahead and do fluffy things all the time even if they want to.
I know it's hard to believe, but Akanishi Jin does actually have a brain.
If what you've just written wouldn't achieve at least a high grade C in an English GCSE exam (finals at age 16), don't post it. Everyone has to start somewhere, but that doesn't mean you have to expose everyone else to it.
Don't inject Author's Notes into your fic. They can wait for the end, or be at the beginning. And never have a conversation with your character during a fic, because that's just odd.
If there is an emoticon anywhere in your fic that's not part of a quoted text message, IM, letter or email then take it the hell out.