Oh good, it's not just me :D. I don't think I've done it for 10-20 minutes; it doesn't take that long for me to get the effect if I'm trying (and sometimes it'll happen on its own when I just happen to be looking at myself in the mirror), though maybe there's a higher-level effect I'm missing out on because I'm not doing it that long. Perhaps because of doing that occasionally, I can sort of create a similar kind of mild dissociative effect even without the mirror, just by visualizing myself or looking at my body, that sort of thing. Basically, to generalize it, your trying to see/consider your body as if it wasn't yours, not just at an intellectual level but at a more instinctual level.

The mirror definitely helps though. To anyone who wants to try it: try looking yourself in the eyes and tell yourself (and try to believe) that that isn't you. Don't just stand still the whole time; move around a bit, tilt your head, etc. Try to open your mind to it. It's an interesting feeling. :)

Here is a thing which, once I've told you, you'll never stop noticing:

Your reflection is always staring you fixedly in the eye.

Old LSD prank: hand tripper a mirror ;)

I read about a similar effect in one of Richard Wiseman's books: Look into a big mirror with no light in the room except for a small lamp behind yourself. After one minute, your face should look bizarrely distorted. (I never tried it, BTW).

This is true especially between two people as well.