As soon as you've decided on the date for your big day, you can send out save-the-date cards to all your prospective guests. These cards are more of an announcement than an invitation, but you shouldn't send them to anyone who might not make the final guest list (you are telling them to save the date, after all).
If you've already chosen a design and theme for your stationery suite, all you have to do is add names and addresses to your save-the-date cards and stick them in the mail. They're usually informal and are often sent sans envelope as a postcard. If you've yet to select a design, send out cards that somehow invoke the spirit or theme of your pending nuptials -- maybe a snowy landscape if you're getting married around the holidays, or a beach scene if you're getting hitched by the ocean.
Save-the-date cards should include the couple's names, the date of the wedding and, if possible, the location. If you two haven't agreed on where you'll say "I do," just give as much information as you can. Mentioning the fact that you're planning to be wed in Virginia or on a tropical island will help your guests make arrangements, even if they're not exactly sure where they're going yet. You should also include the promise of a future, formal invitation and the address of your wedding Web site if you have one.