I didn't get this very much this year but I have had it on various previous years. I just babied myself for a couple/few days, took things slow, let myself be a little "off" and kept my decorations up a bit longer. Then I channeled holiday excitement into Thanksgiving and Christmas.
"Other holidays" channeling has really helped me, I find. I think of Halloween into Christmas as one long "holiday season" that only begins with Halloween. It has definitely taken the sting out of the post-Halloween blues.
I also keep up some autumnal stuff. That has helped too. Again, it makes it all one long(er) season. Autumn stuff doesn't look weird when it's up until Thanksgiving, because Thanksgiving is associated with fall colors too - leaves, pumpkins and other gourds, Indian corn and so on.
I think some blues following the Big Day are natural for some people considering how much time and effort we put into that "one day," then bam, all of a sudden it's gone. Be gentle with yourself, and start climbing out of the blues and planning for next year!