The thing you're thinking of is the expansion tank. Basically, the propane will only evaporate from your tank and flow through your lines at a given rate. You can't really speed that up much, short of heating the tank to increase pressure.
So, to make big fireballs, you let the propane vaporize into a secondary expansion chamber (people often use another propane tank for this purpose since they're conveniently available and can withstand the 150+ psi). Then, you can let the fuel out of the expansion chamber all at once for a big impressive burst of flame. Make sure you use high-pressure (liquid service) hose, since you'll have full tank pressure all the way up to the valve that lets the gas out of your expansion tank.
If you don't want to build one, the bonefire folks (bonefire.com) will sell you one.
Oh, and be careful.