I haven't used a wheel chair, so I can't give you an effective response. However, if you can make eye contact you can then either talk to the person, or scare them away. It's human nature to be curious about something different. If it's a young child staring, they may have never seen someone in a wheel chair before. It's an opportunity to talk and teach. You can say something about how it's hard for you to walk, and the wheel chair gives you your freedom.

I did have a roommate in college in a wheelchair, and he didn't mind people coming over and checking out his wheel chair. It was a very cool one...he got a special hand operated bicycle attachment so he could go really fast in the bike lanes. He wore a helmet, and went faster than a lot of use on bikes.