I have a 2 car attached garage in a raised ranch in CT. My shop is in one corner wrapped around the walls, and I can get both cars in the garage and still work as long as I am not dealing with sheet goods or large pieces of lumber.
I picked up one of the 1500W oil filled electric heaters that has a timer at Lowes for $60. It automatically turns on when I wake up in the morning and stays on for a few hours, then turns on again as I leave work and stays on until I go to bed. I insulated my garage and also have a few of those heated floor mats to stand on when I work.
http://cozywinters.com/shop/tfw.html
I put up the plastic shrink wrap over the windows that adds around 3 - 4 degrees to the inside temps.
With all of this going on my garage stays no colder than 53F and gets as warm as 68F. I find that when I am working out there with the lights on, the lathe on and the DC running, temps spike up around 4 to 6 degrees just from body heat and waste heat from the equipment.
The coldest it has been in the garage was 53F one morning at 5:00 when it was 11F outside, the warmest it has been was 68F when it was 34F outside. It generally holds at 59F.
With a smock over a fleece vest and short sleeves I am quite cozy working out there. As I write this I just checked, it is 19F outside and 57F in the garage shortly before 6:00 AM.