First of all, how are you going to write all those addresses? If you are like most couples, you'll have hundreds of invitations to address. Here are your options along with pros and cons:

Calligrapher:
Pros: You will get a beautiful, formal look that everyone will compliment.
Cons: Calligraphers can be very expensive, and sometimes hard to find.

Write them yourself:
Pros: You can get it done at your own pace- plus, you can have friends and family help out.
Cons: Not everyone who is willing to help has beautiful handwriting, or even legible handwriting at that.

Print them:
Pros: You will get it done fast, with a uniform font that can match your invitations.
Cons: You'll need a good quality printer, or get it done some place professionally to have it look nice.

DO NOT, whatever you do, print labels and stick them on. Not only does it look cheap, but it is much too impersonal as well. Also, it is near impossible to get each label exactly straight, which means needing a lot of extra envelops for mistakes, or a lot of crooked addresses.

Of course, it wouldn't be a wedding task if there wasn't something traditional you had to do. In the case of invitations, you must spell everything out- no abbreviations. The only abbreviation allowed is for titles (Mr., Mrs., etc.). Here's how to do it:

Married Couple:
Mr. and Mrs. Gregory Miller
345 Greentree Drive
Plainfield, Vermont 12345

Couple with Different Last Names:
Mrs. (Ms.) Nicole Hansen and Mr. Frank Roberts
65 Riverbank Avenue
Lewiston, Illinois 23456

Couple with Children:
Mr. and Mrs. Richard Potter
and family
974 Redwood Court
Smithville, Nevada 34567

The inner envelope can be addressed much less formally:

Married Couple:
Suzanne and Gregory Miller

Couple with Different Last Names:
Nicole Hansen and Frank Roberts

Couple with Children:
Janet, Richard, Paula, and Nathan Potter


Whatever you choose to do, make sure you give yourself enough time to address the invitations before your deadline to send them out.