... or don't!
Depending on the situation, pulling over can be one of the worst things you can do. Why? Because most people have a tendency to slow down before they do any kind of maneuver with their cars. Because of this, usually sensible way of driving, a car that tries to pull over when there is limited space on the street actually slows the emergency vehicle down before getting out of the way.
If you see the emergency vehicle coming and there is plenty of room to your right (or left if you are in one of those countries): pull over. If not: keep moving until you find a place where you can get out of the way fast, or the guy behind you (who probably deals with this kind of situation every day) finds it for you.
The ambulance drivers and policemen I have talked to all agree that the thing that slows them down the most is people who panic when they see an emergency vehicle behind them: They slow down and try to stop instead of moving along.
Clarification: I'm not saying you should break the law. I'm just pointing out that there are more than one way of "getting out of the way" and that slowing down to pull over might not be the best one...