I use heat packs from October to mid April. Heat packs can kill though so use plenty of "stuff" between it and the bags. I use two pieces of paper over the heat pack, a plastic liner, and shredded paper in the liner, then I add the bags.
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.