Where you place your goodies in the oven can make a big difference due to the varying temperatures in different areas. The center of the middle rack usually has the most even temperatures. Also be sure to leave at least a few inches in between the oven wall and your baked item. This is important so natural hot air 'convection' currents work to bake your item evenly. If you've got a crowded oven with multiple racks being used, it's usually a good idea to rotate the items between the racks halfway through the baking duration.