1. The wheelchair is a part of the person's personal space.
Handling someone else's wheelchair or hanging items upon it implies a close, familiar, or familial relationship. Request permission before taking such liberty.
2. Introduce yourself, shake hands, and talk normally and directly to the person, not the care giver. Neither stare nor avoid eye contact.
Relax and make an open, friendly, first impression -- just as you would upon meeting any other person!
3. Offer assistance, but don't insist.
It's kind to offer assistance -- just as you would upon meeting any other person -- but do not be pushy if the offer is demured. The person in the wheelchair knows best what assistance s/he really needs.
4. Use normal vocabulary.
Speak normally, just as you would to any other person. I first learned this from our blind neighbor, when I mentioned "seeing" something and then apologized with embarrassment. He laughed and assured me he "sees," just in a different way. People in wheelchairs can "run along" or "walk down the street" or generally progress, too, just in a different way.
5. Sit down, if possible, for an extended conversation.
It is awkward for a seated person to arch his/her neck back over a long period to converse with a person standing close by. Consider sitting down for an extended conversation or, if that's not possible, going down on one knee to speak at a mutual eye level.
6. Answer children's questions matter-of-factly.
If I had a nickel for every child who innocently pointed at my son in his wheelchair and asked loudly, much to their parents' chagrin, "What's wrong with him?", I'd be as rich as Croesus. Children are (sometimes brutally) honest. But, they also respond well to honesty and kindness in return. Simply smile, encourage the child to say hello or acknowledge the individual personally, and quietly explain the difficulty in brief, simple terms. For example, my son has cerebral palsy and other related diagnoses, so I usually just say, "Well, this is [name]! And his muscles don't always work the way they're supposed to, so he uses a wheelchair to get around."
7. Thank the person and consider offering future assistance.
Express to the person how lovely it was to meet him/her -- just as you would upon meeting any other person! If you feel moved to do so, make a sincere general offer to assist in the future, with whatever might be needed.
8. A person with a disability is not a "poor cripple." In many ways, he or she is just like you!
It is difficult enough to compensate for a disability in one's life without enduring excessive and sometimes embarrassing or demeaning pity. Try to see the unique, individual person and not his/her infirmity.
9. If you don't know, just ask!
It is so heartwarming to receive even the most awkward inquiry when it is prefaced or made simply with genuine kindness, interest, or concern. I'd much rather educate someone helpfully than have a person miss an opportunity to feel more comfortable around those with special needs.
10. Afterward, remember the person in your prayers. Pray for his/her strength in dealing with the disability, and thank God for your opportunity to care for others.
On the most elemental level, having or living with someone who has a severe disability brings a whole new level of appreciation to the old adage, "Well, at least you have your health!" I can't speak for every person who is disabled, but I can tell you that my own experiences over the years have revealed that they are just like any other person you'd meet (do you sense a theme here?). It's just that sometimes their cross to bear in life might be more visible than yours.
I encourage you always to reach past your own hesitations and extend a hand of welcome, both literally and figuratively, to anyone you might encounter who has special needs. Even if you don't know precisely what type of help or welcome to offer, your genuine smile and open kindness will be a thousand times more comforting to that person than if had you remained silent in your uncertainty and sidled away.
Who knows? You just might be meeting the friend of a lifetime!