There have been several discussions of using canister stoves on Trailspace in the past. You should search the archives over the past couple of years for a more complete discussion. But here are some of the points.
There are a couple of traditional methods for using butane at temperatures below freezing. For the past 20 years or so, the manufacturers have added propane and/or isobutane, which both vaporize below freezing (propane down to -40F/C and isobutane down to +10F). Some (MSR, Markill, some others) produce canisters containing pure isobutane. The problem with the mixes is that the more volatile gas(es) will vaporize off, leaving the liquified butane behind.
Second, and a safe approach, is to set the canister in a shallow pan of water. As long as the water is above freezing, the butane and other gases will be vaporized, and you can run the stove. Jetboil's bottom cover for their group pot is intended for this as a secondary use, but you can use any flat-bottomed pan that is large enough. This is the only choice for stove-tops that screw directly onto the canister.
A couple of less safe methods (with the manufacturers warning strongly AGAINST their use) involve using copper wire or thin metal strips to wrap around the canister with an end in the flame. The heat is conducted down the copper to the canister. WARNING - it is possible to conduct too much heat to the canister, and for it to thus explode.
Some years ago, several of us (Jim S and I did the most experimentation) discovered that inverting the canister on stoves using a flexible fuel line would allow the pressure provided by the vaporizing propane to force the liquified butane mix to the burner. This was in part inspired by noting that the Coleman X-stoves utilize a hinged fuel pickup that sits in the liquid in the canister, but by some small "accidents" that had happened before those stoves came out. We found that some of the stoves worked very well with an inverted canister and used various jerry-rig approaches. In particular, the Primus Multi-Fuel Stove and its successor Omni series worked extremely well (these burn the butane mixes as well as white gas, kerosene, and other liquid fuels). I and others had discussions with manufacturers at the Outdoor Retailer Show. Coleman's reaction was concern over the non-vaporized liquid butane and flaring and spurting. But we soon realized that the reason the Primus worked so well, with no flaring problems was that they had the generator tube passing through the flame (needed to vaporize the liquid fuels). Primus then made a cradle to hold the canister inverted, and at the same time Coleman modified one of their canister with flexible fuel line stoves to have a generator tube in the flame and a cradle to hold the canister inverted. This past summer Jetboil also came out with a similar stove.
IMPORTANT - For smooth operation without flaring, you must use a stove that has a generator tube that receives heat from the burner, thus vaporizing the liquid butane mix that is being forced through the fuel line of the inverted canister. If you try inverting the canister for any other canister stove with a flexible fuel line, you may get flaring and sputtering.
There are presently several stoves from Coleman, Primus, Optimus, and Jetboil (and I think also Snowpeak) that are designed to use the canister inverted. Use one of those purpose-made stoves.