I would recommend looking at circular replication, where all servers are connected in a ring, and each server is slave to previous and master to the next, so any change in 1 replicates all the way through the ring. I've used this at my work to link 3 servers (one external webserver, 1 internal US wamp server, 1 Asian office wamp server). It was fairly easy to set up and the only problems I've had was when the asian office was not reachable for 2 whole days due to widespread internet outage in the area. As soon as it came back up, the replication resumed.
These 2 links were particularly helpful for me:
as was the replication chapter of High Performance MySQL: Optimization, Backups, and Replication, which also had some different replication schemes you might find interesting if circular is not suited for your needs.