So I saw this earlier and it got me thinking exactly how true it is:
I find it especially true with shows like Friends, The Office and CSI - the only real exceptions that I can think of are:
Buffy - I LOVED seasons 5-7, but 8 is kind of meh, on the other end of it I didn't really care for s2 (it was good but far from my favorite) and even though season 3 gave me a lot of characters I enjoyed I didn't really care for it.
Smallville - This show actually got stronger as the seasons went on. It got a lot better with the addition of Lois, and it grew even more once they introduced Green Arrow and wrote out Lana and Lex. But... there was Doomsday.
Weeds - The show makes it a point to jump the shark at least every other episode but I revel in it. That's the whole point of the show, everything Nancy does makes things worse and it expands with each move she makes.
Medium - This show was truly amazing and my dvr is a much sadder place now that it doesn't have weekly visits from the Dubois family. There was absolutely no decline in quality at all with this show as the seasons progressed and that is extremely rare.