If you are ever at a cocktail party with someone in a wheelchair, find some chairs to place around them so people can sit and chat. I can't tell you how much it hurts my neck to look up at people!
It seems obvious, but hold doors open (and if you have a friend, two doors open are easier to navigate through over one door).
Don't look at your phone while walking. People often walk right into me because I have no height! Especially in crowds.
If you have children, make sure they don't run into our chair, kids often aren't paying attention and when running around don't look and I don't want to crush little feet!
Don't be afraid to offer help, I'm often too shy to ask, but if you see me struggling, I am so grateful to accept your offer!
If you own or attend a business with one or two steps out front (or don't own it but frequent the establishment and want to point it out to management), there are inexpensive portable wheelchair ramps that can be brought out when needed.