Dear 1938: "Reflective insulation" is particularly ineffective in cold temperatures. Check the "FTC Fact Sheet" on the Reflectix web site. For example, they claim the R value for cathedral ceilings (with an air gap) is 7 in summer -- but only 1 in winter. This is because the shiny surface reflects infrared radiation in summer, e.g. from a hot roof or garage door -- which is not a factor in winter.
Fundamentally, "reflective insulation" has virtually no insulation value itself; all of the R values claimed by Reflectix are based on an "assembly" with an air gap and ideal conditions. If you apply Reflectix to a surface without an air gap, e.g. a garage door, the R value is only 1. If you double-up with two layers, the R value is ~2. In short, very little insulation value for the time and money.
Garage doors are difficult to insulate, with all of the movement and vibration. Standard foam board is flammable and cannot be used unless it is covered with special intumescent paint.
The best option would be rock wool panels, which are light and solid like foam board -- but non flammable. Roxul is one manufacturer; their ROCKBOARD® 40/60/80 is available with a foil or white poly surface for a more finished look. This product provides a consistent R4 / inch insulation value, year round. Use 1 or 2 inch thick panels, depending on the level of insulation you want to achieve. Even though Home Depot doesn't carry this product on their shelves, you can order it from your store's Special Order desk. (Builders often purchase products via the Special Order desk.)
You could install it yourself without special tools. You can cut Roxul Rock Board with a sharp knife; Home Depot also sells a special knife for cutting insulation panels that works well. The panels can be mounted to your garage door with standard screws; also run thick beads of heavy duty construction adhesive around the perimeter and in lines across the back. The screws will hold the panels in place while the adhesive dries. Home Depot carries Loctite and Liquid Nails adhesive, e.g. Liquid Nails Model # LN-901 Internet # 202203994.
You should also make sure your garage door is air sealed as effectively as possible, Home Depot sells weatherstrip products for the top, sides and bottom.
If the temperature in your garage is an issue, have you also insulated the roof? XPS foam board is a good option.
I hope this is helpful.