Soaking the stem in water heated to boiling works great. For vulcanite 2-3 minutes is all it takes. Lucite takes longer. The 6 minute recommendation mentioned above sounds about right. Vulcanite stems will usually show some oxidation after being soaked in hot water, so they generally get buffed after bending.
I've also bent a stem over a candle, but the open flame method has its drawbacks. Aside from the bad smell, it's hard to evenly heat a longer stem, especially if the stem varies in thickness. Hot water, on the other hand, will make the whole stem pliable.
I do like bending stems around some kind of form. Anything cylindrical works fine--coffee mugs, saucepans, paint cans. For churchwardens, I usually use a garbage can lid.