In reply to TobyA:
Right. Watch a pro mechanic get a back wheel out at the side of the team car in The Tour.
Make sure the chain is on the 11 or 12. Pros who have a puncture will do this as they are slowing down to stop.
Open the skewer, (maybe open the brake - not always needed) hold the bike upright by grasping the seat tube, bang the top of the wheel with your hand, flick the wheel sideways so the chain doesn't get caught on the right hand side of the skewer (much harder if the chain is futher up the casette) - voila!
Perfectly described here:
I haven't used a tyre lever for years. When the tyre is flat go all round the tyre pinching it so the both beads disengage from the rim. Then push the tyre to the centre of the rim where the diameter is smaller than the edge of the rim.
Stand over the wheel, grip the tyre and force it downwards. When you get to the bottom it should just lift off.
Perfectly described here:
E