With DS, I had my 12 week scan the beginning of May around the same time it was my mum's, Niece's, dad's and sister's birthdays. So I bought happy birthday Nana/Cousin/Grandad/Aunty cards, glued in a copy of the scan picture and signed them from Baby 34. I just told other family and friends in person.
Just found out DC2 is on the way and wanted some more inspiration. Got a bit of time to go yet but my favourite idea so far is to get DS a T-shirt printed with "big brother in training, promotion due Oct 2014", get him to wear it and see who notices!
How did/will you announce yours?