I was looking for some info on this and came up with a couple of links.
Can You Scare a Hawk Away? - Raptors Forum - GardenWeb glass ball to rid hawks???? [Archive] - Pigeon-Talk
Another thing you can do is keep birdfeeders out of your yard and put a plastic owl statue out but move it around daily. Moving it around will make appear to be more life like and not just something the hawk gets used to seeing in the same place like a piece of furniture. If you have persistant problems don't try to harm the hawk because it is a federal offence, instead call your local fish and wildlife office. They may be able to do something or give you more advice. Good luck, I know it's scarey and I watch the sky when I take Roxie out to the bike path.