Here's a little illustration I did recently for a commissioned set of baby shower invitations. They didn't want anything fancy or anything that would cost too much to produce, but did request that the colour scheme be pale pink and that it include a baby in a pram with a Docker's beanie, as they're big Fremantle Football Club fans.
I did the illustration in Adobe Illustrator, over an initial sketch that I did in DeviantArt's Muro (I don't normally do that, but I was playing around with it's new playback function), and the result is that if you're interested in seeing my very messy prelim sketch, you can probably watch it appear here: http://sta.sh/0rkwlrle0nm )
I printed each invitation out on presentation paper, 2 to an A4 page, and concertina folded them into thirds – and finally punched off the corners so they were rounded:
Not the best photos, I was in a rush to catch the post.... I tried to keep them simple while also making them a little different through the folding. Most people seem to stick invitations on the fridge, so something that worked there was the aim – but I wanted to avoid the typical A5-folded-in-the-middle route.