Almost once a day I take a screenshot of something on my screen. Here are the commands I use.
Press the following three keys:
[COMMAND]+[SHIFT]+[4] To capture a screen with crosshair pointer with mouse area selection
[MOUSE CLICK and HOLD] to select the area
[MOUSE CLICK and HOLD] + [SPACE] to position selection
[MOUSE CLICK and HOLD] + [OPTION] to mirror select
[MOUSE CLICK and HOLD] + [SHIFT] to fix one dimension of the selection
[MOUSE CLICK and HOLD] + [ESC] to not capture screen
[MOUSE CLICK and RELEASE] to capture the area
[COMMAND]+[SHIFT]+[3] To capture the whole screen.
The screenshots get dropped into your desktop in PNG format. So if you don't like it change it like this:
$ defaults write com.apple.screencapture location [enter_path_here]
$ killall SystemUIServer
To change it back:
$ defaults write com.apple.screencapture location ~/Desktop
$ killall SystemUIServer