Hey I had this issue last year and I did two things, first last year we had our vision system save images from the field so we could tune in between matches. Then this season we built a vision system that doesn't rely on absolute thresholds.
The full source code is here:
https://github.com/KBotics/2809_vision2013
and I wrote an explanation on my website:
http://kevinhughes.ca/2013/02/21/frc...vision-system/