I was always told that the first thing you need to know about melting aluminium cans is that you don't really want to melt aluminium cans; it's too hard. Find other chunkier scrap aluminium instead. But I'm a sucker so I did it anyway.
The big problem is that there is a lot of surface area on the cans, and that this encourages the aluminium to oxidise. Oxidised aluminium holds on to the liquid aluminium like a sponge, so unlike when you melt lead or pewter the metal does not all automagically drop to the bottom. It needs fluxes, chemicals, special furnaces and the like. Yeah, right.
So, take a load of heatproof bricks or pavers and in a stable location away from flammable stuff construct a sturdy furnace. You'll note that mine is built from ordinary pavers on a mud bank, surrounded by foliage, and looks like it'll collapse at any moment. The trick is to have a long tunnel going into the bottom of the furnace, with a little doorway on top to allow you to put things in and take them out. The chimney takes the sparks up and away from your face and hair, and needs to be narrower than the furnace bottom.
Down the tunnel, you direct a hair-dryer, blower, hot-air stripper (good idea Forrest!), powerful fan or similar. This encourages flames and sparks from ordinary BBQ charcoal at the bottom of the furnace. Do not place the fan up against the tunnel or it may get flamed. If I need to tell you to keep a bucket of water and hose at hand, and warn you not to wet the fan you really shouldn't be doing this.
Back to the cans. Flatten these as much as possible and pack the flattened remains hard into a steel can. This steel can lasts only one burn, so pack in as much as you can.
Now light the furnace. A couple of lit newspaper balls in the tunnel generally does the trick, or poke a blowtorch in there. Once you turn the fan on it's a matter of moments before the whole thing flares up. Once it's going turn off the fan, open the door and rearrange the charcoal so you can put the can inside. Put more charcoal against the can, close the door and turn the fan on again. At this point you realise the value of some peep-hole or crack through which you can watch the inferno.
When the can is orange hot the contents miraculously shrink. once everything in the can is a nice, bright orange, turn off the fan and take it out - I use BBQ tongs. Rattle the contents about a bit, then turn it upside-down on a wide fireproof surface. Squish the pile of dross and molten aluminium streams out. Try to round these up into big blobs so they don't oxidise much when you re-melt them.
Now you have reasonably-sized lumps of aluminium that you can do traditional backyard casting with.