There is an excellent article titled "Tuning an Engine by the Spark Plug.
by Brian Odom", which states in part:
"When tuning a performance engine the spark plug is a important tuning tool. Being that there are many different tuning techniques, I will keep this article about the basics of spark plug reading on a Quarter-Mile Drag Strip (or wide-open-throttle tuning).
When tuning by the spark plug, you must always use a NEW plug to get an accurate reading.
To get a clean plug reading the engine should be shut down after the run and towed back to the pits.
Any running of the engine after that will skew the plug reading.
When tuning by the spark plug, you must always remember there are two major variables at play (air fuel ratio and ignition timing).
To make it easier to determine your on the right track when tuning, make small incremental changes and only 1 or 2 changes at a time."
To read more, go to http://honda-tech.com/showthread.php?t=3063102
Check it out, some good info here.