Use Layer-Based Slices
If you’re getting ready to slice an image for the Web and you still have your layers intact, don’t flatten that image before you slice. Instead, let Photoshop create the (layer-based) slices for you. There are two main advantages: (1) it’s easier. You don’t have to drag out slices—Photoshop does it automatically, perfectly slicing at the size of your layer. But even better is (2), when you create a layer-based slice, you can move the layer and (get this) Photoshop will automatically adjust all the slices to accommodate your move, and it will create a new slice for your layer as well. If you slice manually and move your layer—you’re out of luck—the old slice stays right where it was. Plus, creating a layer-based slice couldn’t be easier. Click on the layer you want to slice, then go under the Layer menu and choose New Layer Based Slice—Photoshop does the rest.