From http://www.ehow.com/how_4612604_writing-college-professor-university-professor.html

Step 1
Use your college or university email. This marks the message as legitimate and not spam. It also gives the prof an idea of who's sending the message. It also saves you from looking uncouth to your prof because of your off-color personal email username. (I once had a student whose address was 'bootiephantum1985@mail.com'. Using an email address like that is not a way to make a positive impression on your professor or anyone else for that matter.)

Step 2
Address your professor directly; don't just launch straight into a request. Examples: 'Dear Dr. Smith', 'Hi, Dr. Jones', 'Dr. Zimmerman, I hope all's well with you...'

Step 3
If your college or university email address doesn't use your full last name (in other words, it uses your initials or some other abbreviation of your name), then make sure your first sentence identifies who you are by name. If your class is large, or taught in multiple sections, you may want to include your class and meeting time as well. Example: 'This is Jane Hasenpfeffer from your Medieval Literature class.'

Step 4
Be polite: Don't make demands, don't accuse, remember to write please and thank you.

Step 5
Be succinct: Make your email as short as you can, your prof is probably very busy and doesn't have the time to read the entire backs story of how you had to miss class because you had to post your boyfriend's bond and get his Camaro out of impound. (That's an excuse I got once.) Just get to the point and politely, respectfully, ask your request.

Step 6
Be specific: This may seem to conflict with the previous step, but it needn't be. Make sure you are as clear as possible about what it is you need to ask of your prof, without writing a novel.

Step 7
If you're going to have to miss class, offer to bring written proof up front, don't make your prof have ask.

Step 8
Close your email with something polite like 'Thanks', 'Thanks for your time', 'See you in class Wednesday', 'regards', etc. Then re-type your first name

Step 9
Grammar and Spell check. You shouldn't write your email as though you are texting your friend. Make sure it's got full sentences, proper grammar, and real spelling. DON'T USE TEXTING ABBREVIATIONS OR JARGON.