D.,
My son is now 16 months old, so he was 6 months last November just as we went into the winter. As well as using the fleece footed PJ's, I used a small space heater with a thermostat. It sat in the middle of the nursery, on our hard wood floor away from everything else. The thermostat was perfect because we could set the dial to just warm enough and it wouldn't get too hot in his room overnight. Two other mothers on my block used a this idea as well, with all the smart precautions, and it was the perfect solution. At 9 months old, we introduced a comfort blanket, "woobie", and he sleeps with that now, but he still doesn't keep it on him, so the space heater will come in handy again. The only other solution is to keep your whole house heated up overnight, which I think is just environmentally irresponsible and certainly not cost effective.
Hope this idea helps.