Learn how to downshift the proper way.

[edit] Steps

Learn how to drive a manual-transmission vehicle first.
Start by finding a backroad or less busy area.
Start driving until you reach about 35 mph in third gear.
With your right foot, start to brake.
When the speedometer shows you are going about 30 mph, press the clutch with your left foot.
While still braking, roll your heel onto the throttle and press it lightly, dropping the car into second gear at the same time.
Keep on the brakes, let out the throttle and clutch.
Keep practicing until it becomes a smooth operation.

[edit] Tips

The pedals need to be close together, with the brake and gas at or near the same height. Unfortunately, most American cars are not set up like this, so heel-toeing will be more difficult.