Norm, Garage door insulation is a fairly easy project and can be a DIY. There is a garage door insulation kit available at Home depot and Lowes. The company, Owens and Corning that makes these kits are reputable. Here is a link to the kit itself- http://www.homedepot.com/buy/building-materials-insulation/owens-corning-garage-door-insulation-kit-122989.html
Don't forget about weathersealing the door, too, or the insulation is a little bit of a waste of time.
I did a neighbor's wood door a few years back. I used rigid blue foam and screwed it to the inside. The use of very large fender washers provided a secure attachment. The foam could also be adhered with panel adhesive. This may not be a fancy looking as some of the other commercial kits...but the blue foam can provide R-values at about 6 per inch.