I'm in Minnesota, and I use Reiff heater bands on the cylinder bases and a pad on the sump. Kennon Covers in Sheridan, WY (
http://www.kennoncovers.com) made some very nice engine, propeller and spinner covers for my 170. I don't think they had made a 170 cover before mine, but now they have the right dimensions, at least for an O-300 powered 170.
On a 40-degree day the head temps are about 130, oil temp about 115, and under the covers, the prop is toasty warm, too. I haven't been through a winter with this rig, so I haven't got experience with colder OATs yet. The preheaters are on a 7-day timer, as recommended by Reiff. It goes on Friday morning and off again Sunday night, because with the shorter winter days I only have time for flying on weekends.
Many people recommend *not* leaving electric preheaters on full-time because it may cause trapped water to recirculate between the sump and the colder parts of the engine, but I was a member of a flying club here (Cessna 152, 172, 182RG, all Lycoming powered) that has done it for decades with Tanis heaters, no problems. Their planes all fly at least weekly, often several times per week. Frequency of use surely helps.