Save the date cards provide the easy way to ensure that no-one misses your wedding, the day of your life. It takes time to prepare the invitations and get them out, and in any case they will normally not be sent until about 3 months before the date of your wedding. If you have made up your minds and set the date, then let your friends and relatives know in loads of time by sending them a save the date card. It's a great way of ensuring that you aren't disappointed because someone you really want to be there at your wedding has booked a holiday of a lifetime on the same dates, and it's a rather nice way of surprising them with your news if you haven't already told them.
We've kept our save the date cards very simple. They are informative and clear, and you don't want to send anything too fancy at this time as there is a danger then it might upsatge the official invitation when it is time for that to be sent out.