You don't need me to tell you how important your wheelchair is! At risk of stating the obvious:

Before you go. Clean and oil all moving parts, check for cracks in the frame and spoke tightness. Either use solid (puncture-proof) tyres or pack a puncture repair kit and two spare inner tubes. I sometimes take a spare tyre. Pack whatever tools your chair needs in your main baggage, because since 9/11 your hand luggage can contain no 'weapons'.

On the flight. Don't tell the crew they can fold the chair and remove the wheels. I tell them they can put the backrest down but that's it. I figure there's less chance of the chair becoming lost or broken if it's in one piece in the hold.

During the trip. Common sense. When you're not in your chair, be sure you know where it is and who's looking after it. After showers or if it gets dirty or dusty, clean it thoroughly! Sand is especially abrasive so same applies after desert adventures or beach breaks!