incrediBILL - 10:45 pm on Jan 8, 2012 (gmt 0)
I had a great sale on my most popular items in December, and did very well with sales. It could be that customers who were going to buy did so in December to take advantage of the special pricing.
There you go, I think you just answered your own question.
I've noticed that the month after I run a really good sale that my sales dropped off as well and most likely for the reasons you've already cited.