The easiest way to remove decals from a car is by using a hair dryer to soften the adhesive. Here's what you'll need:
- A hairdryer
- A plastic putty knife
- Goo-Gone, Goof Off or another adhesive remover
- A clean towel or cloth
Step 1 - Use the hairdryer to apply heat evenly to the decal. Keep the hairdryer moving rather than holding it on one spot continuously. If you have a particularly large decal you may need to work on a small section of it at a time. For most smaller decals, however, you should be able to heat the whole thing at once.
Step 2 - Once the decal has been thoroughly heated, use the putty knife to gently peel up one of the edges. Grab it with your fingertips and gently pull the decal off the car. Because the adhesive has been softened by the heat, it should lift away easily. 
Step 3 - Apply adhesive remover such as Goo-Gone to the area. Allow it to sit for a few minutes, following the manufacturer's instructions. When the time it up, gently wipe away the remaining adhesive.
Step 4 - Wash your car to remove any remaining traces of adhesive remover or adhesive. 
Extra Tips: If your decals are particularly large or difficult to remove, you may also want to use a heat gun. It provides higher heat than a hair dryer, which can help soften the adhesive on the decal more effectively. However, if you go this route it is absolutely vital that you keep the heat gun moving rather than holding it in one place. Otherwise you run the risk of bubbling the paint on your vehicle.
The following video does a great job of demonstrating how to remove a decal from a vehicle with heat: