Getting to know your PSB’s
Say you have a logo with a gradient background and you need to save it for the web. You can slice it out of your design, but that requires precision positioning (a tall order if you need a cross-browser compatible site). You could find the gradient background and hide it, but if you have hundreds of layers in your design that would be a waste of time.
Therefore, the simplest option is to convert a set of layers into a smart object. Right-click on your logo layer(s) in the Layers Palette, select “convert to smart object”. Now its a smart object in which you can double click and it will bring you a logo in a separate file trimmed and ready to save.
You could also edit your smart object and any saves will carry over to the original psd.
In my opinion, slicing is so 2005. While slicing has its role in optimizing repeating images, if you demand precision, elegance, and smaller size, then smart objects is the way to go.