[Eliot Ness]

A method that I've used successfully is to turn the can upside down and slowly heat the outside areas with a small Propane Torch. Once softened up from the outside it will just slide out and plop on whatever you have underneath to catch it and the droppings. A heat gun might work OK for this too, but in either case be careful and don't do this around any flammables.

A safer way might be to buy a toaster oven from a thrift store or garage sale and use the basic method that Terry described above. The toaster oven will minimize the risk of domestic violence if your wife gets a whiff of melted tar coming from her oven

edit: just to be clear I'm referring to the large cans that are metal.