I have one of these, which I carry in my front pocket along with my phone: http://www.hamag.com.au/catalog/item...54/4964212.htm

It's a perfect fit for a credit card/train ticket/folded $20/etc; empty it's only 3/16" thick. If I need to carry more around than that, I'll have my bag with my anyway, in which case I chuck my normal wallet in my bag, but still carry this little thing in my front pocket for easy access to my train ticket.