Recently, I needed a way to have external users connect to my PPTP tunnel (to play CTF) without them having to mess with setting up PPTP clients on all of their computers. I figured that if I could just hand them a pre-configured router, they could connect that router to their ISP connection, and it would relay any traffic bound for my PPTP tunnel to me.
So, I set up a couple of ASUS WL500G routers, flashed to the latest dd-wrt firmware. One router was for my network acting as a server, and the other was for their network acting as a client. I gave both routers their own DDNS URL, and routed both IP subnets to point to each other. This worked like a champ! In testing, whenever they put in an IP of my PPTP subnet, it automatically routed them to it. The Border PPTP Router wasn't any faster on their end than setting up a PPTP client on their computers, but it negates that need--they can connect without messing with PPTP tunnels, because I set that up for them inside the router.
Here are the instructions that I used to set up both of my routers: