I think one of Matthews more important topics from his blog post is:
"When you provide comparison and check magnitudes, they should be instrumental magnitudes, not the catalog magnitude that you assume for the comparison and check stars."
Hmmmm, I think I missed that point in my previous 8 years of observing variables. I will try to do better.
Links:
[1] http://www.aavso.org/forums/variable-star-observing/photometry