Constance mentioned the importance of pre-heating your oven. Alton Brown says that even if your oven chimes up telling you that the target temp has been reached, you must wait at least 20 minutes more before opening the door tp slide anything in.
The reason for this is that the oven chimes as soon as the air inside the oven reaches the target temp. However, once you open the door, most of the heat will escape up. Recovering that temp will take a while, especially if the item you placed is cold and large, thereby screwing up your effective bake time and temp. Now if you leave the oven untouched for the next 20 min after chiming, you will allow the mass of the oven (ceiling, walls, floor) to get good and hot, allowing faster temp recovery after you place something inside.
Hope this helps...
'Never eat more than you can lift.' - Miss Piggy