Basically,
As long as you still have an "off-screen" trainer (OST):
1. Teleport away from before he can battle you
2. defeat a different trainer
3. go back to route of the OST
4. menu pops up, pokemon appears!
5. save the game, and go back to step one, if you want to!
I saw this video a thousand times as a kid. Thought the mew glitch to be fake. I never once got it to work!