I just recently sent out "Will you be my bridesmaid?" invitations to my 4 bridesmaids. They already sort of knew I'd picked them but I wanted them to get a nice surprise in the mail.
I made it funny, full of humour and something that they would get out of the letterbox and have a good laugh over, because that is how I wanted it to be - not too formal, very funny, very much "me".
The front cover I just did a photo of myself holding up a sign saying "Will you be my bridesmaid?" and just took it with the self timer on the camera then had it printed. The inside top cover I asked them whether they would promise to *insert lots of funny things here to do with bridesmaid duties and wedding day* and then in the inside bottom of the card I'd photoshopped the Bridesmaid movie poster with all of our heads, LOL, and did a funny yes/no tick box answer.
They all LOVED it!