I have complied suggested etiquette tips below as reference, but feel free to adjust and modify them to suit your event. Sometimes even the oldest rules are meant to be broken.

FORMAL EVENTS

Step 1 : Identify the Host – Typically the names listed first identify who is hosting (paying) for the event. 

Step 2 : Identify the Request – Formal wedding invitation have one of two requests.  If the event is being held in a place of worship the phrase ‘request the honor of your presence, at the marriage of their daughter’ is used.   ‘Request the pleasure of your company, at the marriage of their daughter’ is used for secular locations.  If the couple is hosting, it is common to use ‘invite you to join us at the celebration of our marriage’. 

Step 3 : Identify the Bride and Groom – Traditionally the bride is listed first and if she shares a lastname with her parents, only the first and middle names are included.  The groom is identified with title (Mr. / Dr. / etc.), first, middle and last.  A more modern interpretation often includes just the first and last names of both parties. 

Step 4 : Identify the Date and Time – The date and time are always spelled out, with the month and day being capitalized.  There is no ‘and’ when writing out the year, a correct example is two thousand twelve, not two thousand and twelve. 

Step 5 : Identify the Location – If the ceremony is at a well-known location the street address does not need to be included.  The zip code never needs to be included on the wedding invitation.

Hosted by the Bride's Parents

Mr. and Mrs. Bride’s Parents Lastname
Request the honor of your presence
at the marriage of their daughter
Bride First Middle
To
Groom First Middle Last

Hosted by the Both Sets of Parents

Mr. and Mrs. Bride’s Parents Lastname
And
Mr. and Mrs. Groom’s Parents Lastname
Request the honor of your presence
At the marriage of their children
Bride First Middle
And
Groom First Middle

Hosted by the Bride's Parents, 
but want to include Groom's parent's names

Mr. and Mrs. Bride’s Parents Lastname
Request the honor of your presence
At the marriage of their daughter
Bride First Middle
To Groom’s First Middle Last
Son of Mr. and Mrs. Groom’s Parents Lastname

Bride & Groom are Hosting

You are invited to celebrate
The wedding of
Bride First Middle Last
And
Groom First Middle Last

OR

Together with their parents
Bride First Middle Last
And
Groom First Middle Last
Request the honor of your presence

Bride's Parents are Hosting and Divorced

(Divorced and remarried parents, both hosting)
Mr. and Mrs. Lastname 1
And
Mr. and Mrs. Lastname 2
Request the honor of your presence
At the marriage of their daughter
Bride First Middle

(If issued by the parent who raised her)
Mr/s. Brides Father/Mother’s First Last
Requests the honor of your presence
At the wedding of his/her daughter
Bride First Middle

(If hosting together)
Mr. Bride’s Father First Last
And
Ms. Bride’s Mother First Last
Request the honor of your presence
At the marriage of their daughter
Bride First Middle

OR

Mr. and Mrs. Lastname 1
And
Mr. and Mrs. Lastname 2
Request the honor of your presence
at the marriage of
Bride First Middle Last

Invitation to Reception Only, 
(Common for destination weddings)

Mr. and Mrs. Bride’s Father First Last
Invite you to join us
In celebrating the recent marriage of our daughter
Bride First Middle Last
To
Groom First Last
Which took place on Date, in Location
Come toast the happy couple
Day of the Week, Month Date, Year
Location
City, State

INFORMAL EVENTS

Get creative!  As long as the important details are included (event request, names of the happy couple, date, time and location) the sky is the limit.  Typically titles (Mr. / Mrs. / etc.) are omitted from informal invitations.

Sample 1

Bride First Middle Last
And
Groom First Middle Last
Invite you to share
In the joy of their marriage
Day of the week, the date of Month
At time
Event Location
City, State

Sample 2

Please share in our joy
At the marriage of our daughter
Bride First Last
To
Groom First Last
Month Day, Year (not written out)
At time
Location
City, State

Sample 3

You’re invited
To the marriage of
Bride First Middle Last
And
Groom First Middle Last.
We hope you will join us
And celebrate our union
Day of the week, the date of Month
At time
Event Location
City, State

Sample 4

Bride First Last and Groom First Last
Invite you to attend
Their marriage
On
Day of the week, Month Date, Year (not written out)
At time
Event Location
City, State

Sample 5

Together with their families
Bride First Last and Groom First Last
Invite you to attend
Their marriage
On
Day of the week, Month Date, Year (not written out)
At time
Event Location
City, State