DO give them personal space. Standing over someone who is sitting down is very aggressive and intimidating. Remember the scary teacher at school?
DON'T treat them like a child. Just because a person's legs don't work doesn't mean their brain broke too. They don't give DLA to the hard of thinking (or the country would be bankrupt in a week...)
DO say please and thank you. How would you like to be bossed about?
DON'T expect the person pushing the chair to move it out of your way. If I'm going uphill, I've got my head down so I can use my bodyweight to move an inert weight twice mine against gravity. If we're going downhill, I'm desperately trying to grip slippery plastic handles before both person and contraption go hurtling into the traffic. Pavements are not flat.
DO smile, nod and say hi. Being stuck in a chair can be very isolating as you're dependant on others for pretty much everything. Small courtesies are important.
DON'T look at me like I'm mad for talking to what you think is a "spazzer" or "retard" like they have a brain. They do.