If you send out invitations with a coulourful bunch of balloons on the front that say "We're having a party!" it's doubtful that your guests will expect a formal affair calling for them to show up in top hat and tails.  An elegantly  invitation is bound to give people the message not to show in in jeans.

Invitation message - where do I start?

Advice:

Who's doing the inviting?  Etiquette is to have the Host's name first on the invitation.
 

An example would be:
Who is hosting the event?  Mr. and Mrs. John Smith.
Who/what is being celebrated?  The marriage of their daughter Melanie to Mr. Richard Black
When?  Saturday, the second of July Where? United Church, Sarnia ON
 

Don't:  include registry info on your wedding invitations.

Gift registries are a good thing.  Make it easy for your gift buyers by letting them know what it is you need. However Gift registry information does not belong on your invitation. DO add it to the bridal-shower invitations or by word of mouth.
 

Proper addressing of wedding invitations.
Most classic wedding invitations have an outer and inner envelope. The outer has the recipient’s street address, a return address, and postage. The inner envelope has less information and should be addressed as if you were hand delivering it; this is also where you would place the name of children if they are invited to the wedding. With contemporary invitations becoming more popular by the day, it is not uncommon for invitations to only include one outer envelope. You will need to read each individual product description carefully to determine whether the invitation you have selected includes both outer and inner envelopes, or outer envelopes only.
 

The return address is printed on the envelope flap and should be the address of those hosting the event. If it is a wedding and the bride's parents are hosting, then it should be the bride's parents' address - names are not commonly used with a formal return address. Please note, guests who are unable to attend or who wish to send their gift before or after your wedding date will use this address to send packages.