This has already come up on several other threads (the search button is your friend). However, again, here's my setup:
It's really easy to see the display even out of the saddle, no vibration, and it sits in front of the hands so little aero loss. I used a nail wrapped in a gear cable outer housing, 7 zip ties, a carefully bent 14 gauge spoke and the garmin removable clip/band. It's called a "Clark Mount". Been working reliably for a year. Couldn't go back to having the computer back at the handlebars.
Doug
http://www.newjerseytriathloncoach.com
http://www.triathlonlog.com