Eyes forward and steer: Focus on what's ahead. Reading the road for hazards and steering accordingly will keep you one step ahead. If you have room, steer sharply from left to right, which will create friction and slow the car. Do not do this at high speed as the car is then liable to spin.
Road search: The surface or incline of the road, can often affect the stopping power. If possible, drive along an uphill section of road. This will assist in slowing you down.
Warn others: Let other motorists and pedestrians around know of your situation. Turn on your hazards, flash your headlights and sound your horn if necessary.
Change down: Shifting down to lower gears will generate engine braking and will allow you to slow down faster. Do not immediately change down into first gear while in fourth or fifth, otherwise your car could suffer mechanical damage. Doing this might also give you or your passengers whiplash, so changing down in sequence is important.
Partial handbrake: If you still fail to slow, steadily engaging the handbrake should bring things to a halt quicker. With your thumb on the release button, gradually engage the handbrake to slow the car. If you feel or hear the tyres lock, ease off on the handbrake and hold it.
Worst Case Scenarios
Handbrake 180: If the road ahead prevents you from slowing down gradually, you will need to act fast. To come to a complete stop in seconds, pull the handbrake and turn the wheel either left or right, depending on what is on the road ahead.
Pulling the handbrake will lock the rear wheels and the addition of turning the wheel left or right, will cause the rear of the car to spin around. Apply full wheel lock and maximum handbrake to achieve a complete stop.
Bump: Using your surroundings to stop the vehicle is a last resort. Gently rubbing against concrete walls or guardrails will damage the body, but will bring you to a stop quickly. If you have no alternative, look for a place to "crash" stop. Wide open places where you can slow by coasting, or somewhere with greenery are your best bet.