13. There is no such thing as a self-lubricating ass - and without lube, it fucking *hurts*. For *more* than a millisecond. The only thing on the human body that self-lubricates is the vagina (and technically the mouth, I suppose), and even *that* doesn't do it *instantly*.
14. On that note, take at least a basic high school health class before attempting to write porn. Better yet, get some background information on what you're planning to write (and for the love of *god*, if you're a virgin, don't write sex scenes unless you've done *extensive* research.)
15. No tears. Unless there's trauma/agony/rape/extremly strong emotions/etc involved, the only liquid I should be seeing in your lemon scene should be coming from the *other* end of the body.
16. And on *that* note, if I see too *much* liquid from that end, you had damn well better rehydrate the poor characters. And you had better *not* 'redecorate' the room they're fucking in. (Seen *this* one...two or three times that I can recall offhand. One Sailor Moon lemon, one Young Jedi Knights, and I'm pretty sure the 'Robin + Impulse' (?) fic - all MSTed on SVAM.)
17. Even if it's a PWP, give it at least a *hint* of plausibility. Such as the characters involved actually meeting up at some point in canon, or the characters actually being *attracted* to each other... (Read: no damn bestiality, since animals generally *aren't* driven to mate with humans...)
18. Please don't type with one hand during the sex scene. The English language is your friend, not your bitch. You can do that *after* you've got the scene done.
19. Some things just should not be going into or coming out of the human body - and not always just because it's horribly *wrong*. Size matters inasmuch as it needs to be able to *fit*. (On that note, keep the penis size to a sane level; it's supposed to be at least *theoretically* capable of fitting in a normal human vagina, regardless of what you're actually putting it in...)