One of the many advantages of having an informal wedding is dispensing with tired invitation traditions, such as having an inner envelope, hiring a calligrapher to address your invitations and using language such as “Mrs. Jane and Mr. John Smith.” However, when stepping away from formal etiquette, you still want to make sure you don’t cause confusion or hurt feelings.
Dispense with “Mr. and Mrs. John Smith” and “Mr. and Mrs. John and Jane Smith.” Both are correct but perhaps too formal.
Put the woman's name first. So write “Jane and John Smith” or “Jane Jones and John Smith.” If you are inviting their children, each child’s name goes on a separate line below the parents’ names. This also saves you from having to list them on an inside envelope. It’s also fine to write “The John Smith Family” or “The Jane and John Smith Family” if you are inviting all the children.
Send separate invitations to partners who are not living together even if they are engaged. If you are close with only one partner who happens to be in a relationship, address the invitation to both partners -- Jane Smith and Burton Jones -- but send the invitation to the person you know well.
Do not feel obligated to allow a friend to bring a guest you don’t know. You may want to talk to the friend before you send the invitation to explain that you just want her, and that there will be plenty of people she knows at the wedding. If you do allow a guest, write “Jane Smith and Guest” on the invitation.
Write out “Street” or “Avenue,” and don't abbreviate the state name. This isn't a rule, it just looks better. It also looks better to put your return address on the back of the envelope.
Use the type of stationery and the stamp to convey the informality of the event, or if you have a theme, the U.S. Post Office has a variety of stamps available online. You should still take your full invitation to the post office to ensure you have enough postage, particularly if you are including an inner envelope.
If your or your future partner's handwriting is miserable, ask a friend or relative whose handwriting you admire to address the invitations. She will be honored.
If you are excluding children -- or anyone -- from the wedding, that is certainly your right. However, it is polite to tell the parents or partners before you send out the invitations.