My garage is really cold. One of the things I requested for Christmas was a thermal leak detector, which basically just tells you the temperature in a small 1-2" diameter spot on a wall or ceiling. I found that the garage doors were where most of the heat was escaping, so I decided that the doors needed to be insulated. When it was about 30 degrees outside, an insulated wall between the garage and the house was reading about 50 degrees from the garage, and the garage doors on the inside of the garage were reading about 35 degrees. Once I insulated the garage doors, the doors were reading the same temperature as the insulated walls even though it's much colder outside than inside my house.
So how did I do it? I just took some insulating foam panels, cut them to size, and wedged them between the frames of the garage door panel. To insulate both doors, it costs about $75 in material (6 panels that are 4ft x 8ft long). I cut the foam board on my table saw - which really isn't the best tool to cut styrofoam, but it worked. After 2 hours, I was covered in Styrofoam pieces but I had an insulated garage door.
It's been a few days and I think I can tell the difference. The day I was cutting the insulation in the garage, I could see my breath while working. I haven't been able to see my breath in the garage since then and it's been a little colder than it was the day I cut the foam boards. Even though I've only had the insulation up for a few days, this is hands down the best result I've gotten with any home renovation project for the cost and time put into it.
One last note, the insulating panels only weighed about 8 pounds, which isn't very much, but our garage door had trouble opening a few times last winter due to frozen rollers. With the added weight of the panels, I had to adjust the garage door springs so that they would have just a little more lifting power. Hopefully, they won't get stuck at all this winter now.