Since Jaime mentioned Romance Divas, I'm going to jump in with this. This was a thread started over there. I've changed it around a bit and added my own, cause I think they can be helpful.
1. Don't give your hero/heroine a pet. I did this once, the hero's opening scene was with his dog. About halfway through the book, I realized I'd never mentioned the dog again. Oops.
2. Be careful mentioning specific days or dates because there is a good chance you will forget. But I can guarantee someone else won't and they'll call you out on it.
3. Remember where you've put you're secondary characters. You can't send the hero's best friend out of town only to have him appear in the next chapter.
4. On a continuing thread to #3., why have secondary characters at all if they are only going to disappear in the second chapter? I've seen this happen far too frequently and wonder why bother.
5. A good critique partner is worth her weight in gold. Seriously. Writing can be a very lonely business and while your family can sympathize, they can't truly understand. Your crit partner will not only make you a better writer, she will be your lifeline, your bitch buddy, and if you are lucky, your best friend.
6. Your editor is always right. Well, not always right, lol, but she is not your enemy. If she is asking for changes, there is probably a reason. Listen to them and if after, you still think she is wrong, calmly explain why. Your editor wouldn't have agreed to work with you if she didn't love the manuscript and she is only trying to make it better.
7. Always be professional. Not everyone is going to like your story. Telling them why they are wrong (even if they are) is only going to bite you on the ass.
8. Readers are allowed their opinions. You are not. I know this doesn't seem fair (and sometimes, especially when it's your work being trashed, it's not), but it's the way this business works. It's always best to just shut your mouth and smile.
9. Sometimes readers just don't understand the industry. I got some flack with my last book because it wasn't long enough. What most didn't understand was there was a word restriction, I was only allowed a maximum number of words. I see readers complain about sex scenes being too graphic in erotic romance, complaining about the pricing of e-books, book covers, blurbs, book formatting, editing, where it is being offered, etc. Many of these things are beyond an author's control, so back to #5. You can commiserate together. :)
So what have you learned in this business they call writing? I'd love to hear what you have to say so add it to the list.