Spell Check Your Subject Lines
Whenever you are composing a new message, be it email or a post to a message board, odds are your body is spell checked for you before submitting it. Whether you have a browser plugin or external program handle it, any misspellings are usually caught. However one field which is, surprisingly, not typically scanned is the subject text.
Considering this is the first thing your reader sees, making sure the spelling is correct is important. A simple way to make sure it is checked is to include the subject either as the first or last line of your message and then the spell checker will pick up any errors. You can then move the text to the subject line. Additionally, if you use Firefox 2.0, you can right click on the text box and select "Spell check this field".