If you have a propex or carver heater, you may, like me,
want to increase your propane capacity in order to increase
the amount of time you can camp remotely without a refill.
As others have mentioned in the past, you can buy a fitting
made by Marshall and available through any RV place that
bolts onto the tank and regulator, and allows you to hook an
auxillary tank to feed the system.  It's call an
extend-a-stay valve, or something like that, and costs about
$40-$50 depending on what hoses you get, etc.

The tank i got is what i really like...it is significantly
smaller than your typical barbacue propane tank, but it
holds 10 lbs. of propane, same as the westy tank.  I got the
tank at Home Depot for $24 (they also sell a 5 lb. tank).
The tank is perfect because it is small and yet it holds all
i will really likely ever need during any forseeable winter
camping trip.