This is something we are doing; here are a few possible suggestions. I guess you can have more informal wording if you wish.

Save the Date
for the wedding of
Tiny lastname + Gary Surname
June 24, 2006
Beaucheif Abbey, Sheffield
Invitation to follow

Tiny lastname and Gary Surname
are getting married!
SAVE THE DATE
June 24, 2006
Beaucheif Abbey, Sheffield
Invitation to follow

Please Save the Date
Saturday, the twenty-forth of June
for the wedding of
Tiny lastname to Gary Surname
Invitation to follow

I think the invitation to follow is important as otherwise some people might think that it’s the actual invitation.