I heat the tubing with a propane torch until I can see it "discolor"...turn a different shade from original, but definitely not red. Once it has naturally cooled off I can use a Dave Brown Products mandrel bender to form my bends.
I build my own fuel tanks for diesel powered projects out of tin and using this method works very well.
I am talking about bending either 1/8" or 3/32" brass tubing from K&S.