Spell out "and" instead of using an ampersand (&). For example, write "Mr. and Mrs."
If you're having a formal religious service and using the phrase "honour of your presence," "honor" should be spelled with a "U"—the more traditional, British version.
Check and double-check all spellings.
Avoid nicknames. The bride should be listed as "Patricia," not "Patty."
If the groom is a junior, you can write it as either "Jr." or "junior."
Double-check all proper names with relevant parties.
Be on the lookout for typos and the duplication of words.
Make sure you've got the correct date. Add the day of the week and the year, if the wedding takes place in a different year than the one in which the invite is sent.
Spell out the time (use "four o'clock," not "4:00") and the state (write "Vermont," not "VT").
Read it out loud slowly, then read it backward.
Have someone else proofread it, too.