There are several definitions of proper wedding etiquette. Ultimately it depends on the preferences of the bride and groom; however there are some general guidelines that should be followed.

The Basics: Composition of Your Invitation

An invitation for a less formal wedding can take some liberties with these rules, but for a truly formal wedding, you should remember the following list when composing your invitation:
Write out all names in full, including middle names. It's preferable to omit the middle name completely than to use just an initial, though.
Use the British spelling for "honour" and "favour" unless you prefer to use the American. If you do, use it consistently.
Spell out ALL the words, including numerals in the hour, date and year and all the words in the address including Street, Avenue and Road.
Use Roman numerals rather than numbers in names - i.e., III rather than 3rd
If the ceremony takes place in a house of worship, the wording should be "request the honour of your presence". If it is being held in a non-religious setting, use "request the pleasure of your company".
A formal invitation should be engraved or printed in black in an engraved or script type style.
Traditional formal invitations do not have illustrations, and are printed on white or cream colored card stock. Invitations for less formal weddings may break these rules.

Wording the Invitation

Many years ago, most weddings and receptions were hosted by the parents of the bride, so the wording for the invitation was easy. These days, the wedding is just as likely to be hosted by the bride and groom, by the groom's parents, or a combination of the various parties. In addition, parents may have divorced and remarried, adding yet another layer of complexity to wording the invitation without offending. Here are samples of wedding invitations under various circumstances.

Invitations from the parents of the bride:

Mr. and Mrs. Daniel David Trudeau request the honour of your presence at the marriage of their daughter Lisa Diana and Mr. Leon Peter Davis…

Invitation from the couple:

Lisa Diana Trudeau and Leon Peter Davis request the honour of your presence at their marriage on …

When the groom's parent's host:

Lisa Diana Trudeau and Leon Peter Davis request the honour of your presence at their marriage on …

When both sets of parents host:

Together with their families, Lisa Diana Trudeau and Leon Peter Davis request the honour of your presence at their marriage on…

When the bride's divorced and remarried parents host together:

Mr. and Mrs. Steven Smith and Mr. and Mrs. Daniel Trudeau request the honour of your presence at the marriage of Lisa Diana Trudeau to Mr. Leon Peter Davis

When the bride's remarried mother and her husband host:

Mr. and Mrs. Steven Smith request the honour of your presence at the marriage of Mrs. Smith's daughter Lisa Diana Trudeau to Mr. Leon Peter Davis…

Assembling the Invitation

Wedding invitations consist of several pieces. The envelope should always be addressed first, preferably by hand, to avoid injury the invitation inside. Once the envelope is addressed:
Put the invitation and its protective tissue into the envelope, folded edge first with the front of the invitation facing the back of the envelope.
Place the reception invitation on top of that, also facing up.
Slip the response card under the flap of the addressed response envelope and place it into the large envelope with the invitation facing up and the envelope facing down.
Add any other enclosures such as maps or directions last.
Mail your invitations at least six to eight weeks before the wedding.
Mail from the post office and ask that the invitations be canceled by hand.

Addressing the Envelopes

Finally, a quick guide to addressing the inner and outer envelopes of your invitation"