I've seen many folks use those cupholder mounts like was mentioned. The kind that's got the two things that squeeze the scanner like a vise. THen you gotta hit a button on the side to get it to let go.
I've never used one myself, but I think it's a great idea.
BTW: If you've got a 396T, or other scanner with the little mushroom shaped knob on the back, a standard CB radio microphone clip will hold it. You've gotta bend the mic clip a little bit to get the thicker knob to fit, but you get the picture.