For GenCon 2007 I costumed a DocWagon team that made an appearance in the Shadowrun LARP run by my friend Aaron Pavao. It was an amusing challenge, since most of the costume pieces were modern clothing.
Alan very kindly drafted up a fantastic vector version of the DocWagon logo (size one and size two), which I printed up on black t-shirt transfers. Then I made patches using some black broadcloth. I did big symbols for the vest backs and smaller circles for the hats and shirts.
The other costumees provided their own polo shirts and slacks, but I got the hats (on sale at a local department store) and vests (online military supply place). Ben also let me tack a logo onto one of his small travel bags for an instant medkit.
I think the final results came out pretty well. I always feel a bit of a boost when people stop me randomly at the con and ask me where they can buy the things I'm wearing. ;)