Since I fly anywhere from sea level airports to those with elevations in excess of 6,000', I have developed the habit of setting the mixture for the appropriate density altitude as the final step before adding power for the takeoff.
This final step becomes very easy if flying the same airplane every time, and I confirm the mixture setting by referencing the EGT's shortly after takeoff. I also lean during the climb to maintain the same EGT I would have had if I had taken off at sea level with the mixture full rich.
FWIW, I have been an advocate of WOTLOPSOP for the past 1,000 hrs on my IO-520D with great results as advertised by the GAMI folks.
