// Set proper filename for screenshots
- Change the date format to 24 hours, if applicable, by customizing the Medium times format under (remove all spaces too!)
    System Preferences > Languages & Text > Formats > Times > Customize > Medium
- Run the following command
    defaults write com.apple.screencapture name "screenshot"
- Open terminal and enter:
$ sudo su
$ cd /System/Library/CoreServices/SystemUIServer.app/Contents/Resources/English.lproj
$ plutil -convert xml1 ScreenCapture.strings
$ nano ScreenCapture.strings

change
  <key>%@ %@ at %@</key>
    <string>%@ %@ at %@</string>
  <key>.</key>
    <string>.</string>
  <key>Screen Shot</key>
    <string>Screen Shot</string>

to
  <key>%@ %@ at %@</key>
    <string>%@_%@_%@</string>
  <key>.</key>
    <string></string>
  <key>Screen Shot</key>
    <string>screenshot</string>

then
$ plutil -convert binary1 ScreenCapture.strings
$ killall SystemUIServer