Since we finally got a cold snap down here in the South, I thought I would go ahead and insulate my garage door. My garage is the standard double garage with approximate dimensions of 7' x 15'. I used 1/2" Foil backed EPS Boards common at your big box hardware store. I also glued scrap pieces together (1" x 3" roughly) to create 1" thick sandwiches. I glued the sandwiches on all four corners of each board to maintain close to a 1" air gap between the boards and the garage door. Overall, this was a easy project and I am quite happy with the results. Here is a picture of the (almost) finished product:
If all things were perfect then I would be achieving an R of 3, which I know I am not. I do plan on putting boards on the garage panels around the windows, without covering the windows (I like my natural light too much), in the next couple of days, but I just didn't have time to get to it last night.
The total cost, not including glue, was just under $30 (which covers the entire garage door). The wife likes the aesthetics (big plus), the cost was right, and my garage seems warmer. This morning my thermometer read 12 deg F ambient and 35 deg F in the garage. I am looking forward to having a cooler garage in the summer since it is a Western facing garage door.