Like Pete (Gilldiver), I use a hot water rinse followed by drying with compressed scuba tank air.
I find the tank will dry more quickly and evenly if the drying gas is injected through a long perforated tube which has been inserted into the tank.
The tube is made from about 20 inches of 3/8" ID polyethylene tubing into which I've drilled about forty 1/8" holes in the sides and one hole in a wooden plug placed in one end of the tube.
Since my tap water is quite high in minerals, especially iron, my tanks were prone to light flash rust, even when dried quickly.
My compromise method is to turn my hot water heater up and rinse the tank with about 140F tap water, followed by a quick light rinse with about a quart or two of distilled water sprayed under high pressure from a fire extinguisher through another perforated tube.
Then the tank gets a dose of an 8 oz solution of Compound "O" rust inhibitor, followed by agitation or rolling on the tumbler for 3 minutes, then drained. It is then quickly rinsed with distilled water for 5 to 10 seconds, then blasted with my perforated air wand for about 3 minutes, using up about 15 cubic feet of tank air.
Since my tank has cooled off quite a bit, it doesn't dry as fast as Pete's arrangement, but at least I'm keeping the flash rust at bay!
Dave C