1. Use graphite only. Oil damages the paint and collects dust. I'm told that the graphite works better than the new white Teflon.
2. Break in the wheels by spinning them with lots of graphite. 
3. Right before check-in, re-apply graphite to the wheel center around the axel shaft.  Use a decent amount of graphite, but not too much.
4. Put a small drop of white glue or paint where the axle goes into the car body and put powdered graphite on it there. That causes less friction if the wheel should rub against the car body.
5. The majority of the friction and therefore drag on the vehicle will be at the wheel to axel interface, so good amount of graphite here is important.