Never push someone in a wheelchair without permission. Always ask. If someone just pushes my wheelchair, they are literally taking control. Imagine you're driving your car and suddenly someone hops in through the window and takes over the steering wheel, gas, and brakes.

Also, a lot of people recommend that when talking to someone in a wheelchairs you get down to eye height, e.g. by sitting down or crouching. I'm not sure if there are any people who actually like that last thing, but I don't at all. Crouching when you talk is something you do to a small child, not to an adult even if he's in a wheelchair. By all means sit down, or stand more relaxed instead of 'at attention' to make yourself that bit smaller and and tower less over them.
Also, realize that when you're standing in an aisle and I'm in my wheelchair in an aisle, we're both "standing". So if you're sitting down to ask me whether I need help picking the milk from the highest row, it feels forced.

A fun thing that happens sometimes at school is that when I use my wheelchair, people can be so helpful it only inconveniences them. Like we're sitting in the hallway during break, and I say 'shall I throw your wrapper in the bin, too?' They often insist on throwing both theirs and mine.
It's sort of hilarious, because they need to stand, walk to the bin, turn, walk back, sit. While I only need to turn my hoops twice and turn them back twice to ride back. :-) It's actually easier for me.

So 3 guidelines:
- Never push without permission
- Towering over someone can be a good thing to avoid, but a lot of adults with wheels feel it's denigrating to crouch to eye height
- Ask if you can help, don't take over
And: if you have an acquaintance in a wheelchair, just ask how, and in what way, they like to be helped.

By the way, about the crouching. If it feels natural in a situation, that's different. In November my teacher crouched to talk to me about my grandmother who died the night before. The crouching was natural then, because I was upset. He'd probably have crouched if I'd been sitting on a bench, too. If he'd ask me whether I found the homework difficult, or explained something I'm interested in but not on the curriculum, crouching would feel weird and unnatural. Get the difference?

Some people with disabilities like to use or avoid certain words. E.g. I often refer to 'my wheels' or to 'using wheels instead of legs for a few days'. You may copy me or you may not. Just do what sounds natural.

Asking is better than assuming. But there's a difference between asking a stranger on the street (don't, rude curiosity), asking the guy next to you in the bus after you've discussed the weather (can be acceptable as long as you don't intend to spend the rest of the trip interrogating me), asking a classmate you're acquainted with (sure, although take the time and place into account and don't make too big a deal out of it) or asking a classmate you sit next to often (acceptable, see previous line). (I'm assuming that with a friend, the subject has come up before - or hasn't for a reason) They don't owe you answers, don't demand them. Also, the events leading up to being in a wheelchair frequently contain bad memories: people might prefer to stick to "I was ill". Or go into way too much detail
Or they might not know the specifics of their condition; possibly because the doctors don't, either.

Don't be overprotective. First time I was out to the grocery store, a few months after my amputation, I was wearing shorts. Child came up: "Why do you have a silver leg?" I've never seen a mother look so horrified. ;-) I didn't mind, I just explained that my leg was very sick so I got a new one from the doctor.
Similarly, if someone makes a funny joke to me about my wheelchair, you don't need to tell them off unless you personally know me and know it bothers me. If I make a joke, you can 'up it'.

Just be natural!

In case you're wondering, I'm 20. Crutches at 12, chair at 13, amputation at 15. I use my wheelchair on average 3-4 times a year for 1-2 weeks I think, not counting when I just use it over the weekend or something.