Some of this instructions may make no sense because they refer to a picture that pontifex gave me via PM. I don't want to post the picture without his permission, so....
When working with the magic wand tool, it is often necessary to use it in conjunction with the other tools, such as the rectangular, circular marquee tools and the polygonal wand tools.
Two very important keyboard commands to know is alt and shift.
When you press and hold alt after making a selection, you'll see a little minus sign next to your pointer. This allows you to subtract from the selection.
When you press and hold shift after making a selection, you'll see a little plus sign next to your pointer. This allows you to add to the selection.
When you select the magic wand tool, there will be a few options at the top:
Tolerance: The magic wand guages color differences in order to make its selections. If you're selecting something entirely white on a black background, the magic wand will easily select all the white because there is such a large color difference between black and white, and it's easy for it to guess that you want to select that color. It gets harder when you want to select a white area that has, say, light gray areas or has white spilling into an object, such as Clint's hat. If you use the magic wand on the white border, it'll select part of Clint's hat because the color is so close to that of the background. A lower tolerance makes the wand more picky about which colors it'll select. A higher tolerance will make the wand less picky about what colors it'll select.
Anti-Alias: Smooths out the selection, gets rid of jaggies.
Contiguous: will only select colors that are physically connected to each other. If "contiguous" is unchecked, selecting on a white area will select ALL the white areas in the picture, including the white on the men's shirts.
Sample All Layers: You only have to worry about this if you've got a multi-layer image with the layers overlapping each other.
Tips:
Like stated above, you often have to use the wand tool in conjunction with the other tools, such as the rectangular, circular marquee tools and the polygonal wand tools to make your selection exact.
Say you use the wand tool to select the white quadrant to Clint's right. You'll notice that part of his hat was selected. To fix this, you can click on the lasso tool, hold down alt, and subtract from the selection that flooded out to his hat. You can also use this method with the other selection tools like the circular marquee and polygonal wand tools.
If you find an area where the magic wand didn't select enough, use a selection tool and hold down shift to add to the selection.
The magic wand tool is only for general, quick selections. To finish off the selection so that they're perfect you have to use the other tools in conjunction with the alt and shift keys.
Lastly, remember that in this case, you're selecting the white area, which is actually not what you want. After you are finished selecting, right click on the image and hit "select inverse" so that Clint and his gang are selected, and then "Layer via Copy."