Miranda:

I have one of the Garmin Vista HCx units that I ride with from time to time when i'm going to be way out in the piney woods. I've got the Garmin handlebar mount you were talking about, and haven't had much trouble with the unit working loose. But as a little extra measure, most of the Garmin handheld units come with a short "dummy cord" that you can wrap around your wrist; I take a couple of loops with mine around the handlebars before I snap my "varmint" into the bar mount. If it does work loose, the dummy cord should keep it from hitting the ground and being left behind.

Tom