You can paint white on the outside of garage. White reflects a lot of heat as compared to other colours.
Reflective roof coatings
If your garage has its own roof then you can install reflective roof coatings. This will reflect heat rays, eventually cooling down your garage. Consult a roof contractor for installing reflective coatings.
In order to let the hot air out from your garage, you can install a roof exhaust vent.
You can plant trees and shrubs outside your garage to lower the inside temperature. When these plants grow, they will block sunlight from falling onto garage. Remember that you can get plants which grow in very little time.
Evaporative coolers or swamp coolers are less expensive than air conditioners. They also generate lower electricity bills. You can place them inside your garage in order to lower down the temperature by almost 20 degrees. The fan in these coolers draws in hot air and when the water evaporates, it cools down air. Remember that these coolers also keep your house cooler.
Air conditioning unit
You can install air conditioner with exhaust vent inside your garage. Remember to get smaller unites in order to keep your bills lower. Smaller units consume less electricity.
Cool your car before parking
You should not park your car right after driving it home. Let it cool before parking inside your garage. In case you park the car immediately inside garage then the heat radiated from your car will add up to the heat present inside the garage.