Here are my taxi bugbears -
Fully close the throttle before applying the brakes. If you are rolling downhill and getting faster and faster then its probably because you still have the throttle open, close it!
Don't open the throttle to Max RPM to get moving, 1200 is sufficient on most light singles on tarmac. If it isn't then add the power slowly until you start moving then return it to 1000 RPM.
Don't taxi along riding the brakes on and the engine screaming away. Don't release the park brake without closing the throttle first.
Don't sit there watching the control column bash against its stops, hold the bloody thing still in the correct position for the wind. Don't "steer" with the control column and for god sake don't lean into turns like you are Nigel Mansell!
On a narrow taxiway don't try to turn the aeroplane around on itself using full power, full rudder and jerky max differential braking when there clearly isn't the space and even less point. Accept the fact that you won't be facing into the wind for your power checks and you will be depriving the engine of 5 knots worth of cooling airflow when you've just have buggered the engine anyway getting round into the position.
When doing the prop check on a VP, don't slam the prop lever back and forward against its stops in half a second.