Here's a couple of things that perhaps most of you already know, but I found by accident and are quite handy.
Rescuing Blown and Blocked Areas
Sometimes, in the process of editing an image you realize too late that some small area has degenerated to pure white or pure black and can no longer be rescued. There are a couple of things you can do. First, you can select all the image, copy it, then use the history palette to revert to the original image by clicking on the image above the top of the list of steps, then paste back your edited image on top. You can use a white mask in this with a small area painted black to rescue the blocked area. You can even put adjustment layers between then to adjust the area you want.
Some times even this isn't enough and you need to go back to the raw file. In this case, assuming you have saved your edited image, you can reopen the original raw file, edit it in whatever raw processor you want, ignoring all of the image but the area that ended up blocked, adjust to make it as good as possible and then open the image, copy and paste that on top of your edited image using a black mask painted white in the relevant area.
Should you have done some cropping in the mean time, you will need to align the images but since they are the same file, this is easy using the move tool at the top of the tools palette.
Controlling Where A Layer Takes Effect
There are times you have defined an area you want to adjust with a masked adjustment layer, but then you realize you need to do more work on just that area, whether it be another curve or perhaps a saturation change or whatever. If instead of picking a new adjustment layer from the icon at the bottom of the layers pallete, you use the layers menu to create a new adjustment layer, you are given the option of using the previous layers mask to control this one, clicking that on means this layer only affects the same area of the image as the last layer.
I imagine there must be a way to do this from the layers pallete and perhaps some nice person will tell us what it is.
There are a million tricks to Photoshop, it's just a matter of finding the dozen or so that are most useful to the way you work.