One thing I never used to pay attention to is gamma correction. This is mainly because it rarely gets mentioned, and also because you can usually get pretty good results without ever even thinking about it. However it only took a few days at my new job for me to realize just how essential it is if you want professional-quality results.
I was playing Killzone 2 the other day, which reminded me of the lens flare trick they used.
Today I have two XNA samples fresh out of the oven: a Motion Blur Sample, and Depth Of Field Sample.