Tip: Copy and Paste within the App Inventor Block Editor
This is not obvious but … you can select a group of blocks and press Ctrl-C (on Windows) and then press Ctrl-V (on Windows) to copy and paste your blocks.
Press Ctrl-X (on Windows) to delete a selected group of blocks. If you did not mean to delete those blocks, press Ctrl-V and paste them back!
Another trick is to select a block or blocks and then press right-click on your mouse (or Ctrl-click on Mac OS X) and then select Duplicate from the popup menu. Copy and paste should also work on Mac OS X using the standard Mac OS X keystrokes but I have not yet tested it there.