One thing Quake does is send out packets 10-20 times a second with the
player's position etc, this is good for games like Quake, so if you get a
bad lag, your player doesn't carry on moving and end up in the Lava!! 
Obviously you wouldn't need to do that many packets a sec, maybe 1,2 or 5
a sec maybe; which would keep the bandwidth down- Quake was originally 
designed for LANs not the net, they made QuakeWorld so Quake was more 
playable on the net, and it reduces the bandwidth used so you can have 
more players- up to 64 now with MegaQuake. 
  Dave had a good idea- you could send out the player position packets
Unreliably- this could be useful if your game is sending out lots of them
per second, so it wouldn't matter if the odd one didn't arrive. Obviously
you'd still need to send collision messages, system messages and powerup
messages etc reliably. By sending out most of your packets Unreliably you
would help cut the bandwidth needed by about half! This would work well in
Driving/Flight Sim type game, but may not work so well in a constricted
maze game- you'd have to experiment.