1. Keep it short.
2. Keep it short. If you had 100 girls wanting to sleep with you, would you spend more than 5 minutes writing an email to some girl you just met? If you find that your emails are longer than hers, you're writing too much.
3. Don't ask a bunch of questions. (it makes you come off as needy, and makes your email annoying to reply to.)
4. DON'T ask stupid boring questions like "how has your day been".. That will make her groan and think meh I don't feel like answering this email now.
5. Put yourself in her shoes and try to make the email easy to answer. Don't ask philosophical questions about the meaning of life.
6. Keep it funny, don't talk about anything serious. So NO, you don't confess your love to her.
7. Don't say stupid AFC shit like "it was nice meeting you". She must wonder whether or not you're into her, saying you're into her is like telling somebody what happens at the end of a movie.
8. If you DHV, don't brag and say stupid shit like "I go to the gym every day, yep it's good to be strong and healthy and have a 6-pack". Keep it indirect, and try to make it relevant to what's being discussed.
9. Try to refer humorously to something that you talked about when you met. Make her laugh and remember why she gave you her email in the first place, but at the same time don't recycle the same joke. So for example if you had told her that her nose looks like a puppy's nose, you can email her and say "you know what, I went home and took another look at my neighbor's puppy, he definitely has a cuter nose than you!

"
10. Don't email too much, lest you become an e-buddy. At some point, get her number and ask her out, the sooner the better. Keep in mind that if you meet and it turns out she's not interesting or if she's not going to sleep with you, you'll have wasted a lot of time if you sent 10 emails back & forth with no results.
EDIT: (Note: most of this applies to text messages as well!)