Just keep your MacBook closed when plugging in the external display (or wake it up with an external mouse or keyboard), and the internal display will stay off when you open it later (if you need to use the internal keyboard).

This doesn't work anymore under Lion. I now have to use a small magnet to trick the lid-closed sensor.

Either cut-and-paste or a dual pane Finder would work for me.

Cmd-Opt-V (vs. just Cmd-V).

sudo nvram boot-args="iog=0x0" still works on 10.8.