There's a way to change this through using that software Cocktail. Otherwise, you can while taking a screenshot add in the shift (or ctrl?) key and it will copy to clipboard instead of making a new file.
To change the default location for screen captures so they get saved in a specific location instead of cluttering the desktop, you need to use the defaults command in the Terminal to change the path. It can be done like this:
Create the folder in which you want the screen captures to be saved in the location of your choice.
or if you are not comfortable with the terminal, as a lot of people aren't.
i suggest Onyx http://www.titanium.free.fr/pgs/english.html which not only allows you to tweak your sytem to just how you want it, but also runs maintenance to keep your mac running as best as it can.
i have heard Cocktail (as suggested by Goobimama) is a very simliar app, but i have yet to try is out.
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