Hitting the side of the NES works for me most of the times (seriously)
When it does it's little flashing screen/flashing power button, hit the side of it, and the title screen will pop up.
Doesn't always work though.
And for those of you with a SNES, if it has trouble playing a game, wedge a folded bit of paper into the cartridge slot in front of the cartridge. Works everytime.