Here are some tricks for working with layers in Photoshop CS5 if the Layers palette gets too crowded, or if you need to apply the same filter to multiple layers, streamline your Layers palette, or work with multiple layers as a single entity:

To: Do:
Merge the active layer into the layer below Command+E/Ctrl+E
Merge all visible layers into the active layer Command+Shift+E/Ctrl+Shift+E
Merge a copy of all visible layers into a new layer Command+Shift+Option+E/Ctrl+Shift+Alt+E