I usually find the 2 colors that I want to use, then I go in and select the gradient tool and choose how I want the gradient to go. Weather it be a fade out from one color to the other, or a fade from one color, to another, then back to it's original. Those are my most commonly used ones. From there, I would take either the eliptical tool, or the box or shape creating tool and make a white shape above the gradient on a new layer. Rasterize that layer, then maybe take the eraser tool, bump up it's size to be about a 1/4 or more of the size of the shape, make sure it's what I call the "fuzzy" kind of eraser, where it fades out on the sides. Then erase however much I want of the rasterized shape that's above the gradient, and change that layers opacity to a nice ghostly like shape. Viola! I'm sure there's about 100 different ways of doing it, but that's how I usually do it.