Hmm... well you want it to have the best possible view of the sky so I'd suggest as far forward on your dashboard as possible. Since its kind of out of the way there anyway, I'd say velcro would probably work and not be too unsightly...

you could also put it under your rear window, since that would help the visual appeal (or lack thereof) since the bluetooth range should easily extend to the rear of your car unless you drive a Ford Expedition

If you're not planning to move it around much, I'd suggest double-sided masking tape or double-sided scotch tape. Then the tape would be invisible and the unit shouldn't need to stick much to keep from sliding around