Roseanne: Was a great show prior to late 1995, then jumped the shark bigtime. Part of it was the absurd new Becky, part of it was the increasingly outlandish and vulgar plots, part of it was the admission that it was all "fantasy." I think part of the reason for its jump was how outmoded these "socially conscious" working-class sitcoms were getting after 1995 or so.
The Simpsons: Jumped the shark in 1998...bigtime. This was a slow process that was going on since late 1996. It can be chalked up to the show becoming sillier and running out of clever plot ideas, the writers leaving, etc. Part of it was also the desire to keep up with South Park's vulgarity which was stealing its audience...SP was like a slap in the face to OFF. So it tried to bridge the gap and failed miserably. And then, it started relying on the special guest stars instead of using them to supplement the show.
The Fresh Prince of Bel-Air: Like, late 1993 with the opening of the 4th season. The old Aunt Viv left and then Will Smith got too much control, so it all became about him shagging chicks and not the family, and the excessive special guest stars. Also, the show became a little bit of a relic of the early '90s by the time 1993-1994 rolled around.
Seinfeld: Never. It was the heart of the '90s....well, until the finale. I admit it did peak around 1993 or so, though.

Thoughts and additions?

   Northern Exposure jumped after the casting change in the final season...

   Exit - Rob Morrow (Dr. Joel Fleishman)

   Enter - Paul Provenza (Dr. Phillip Capra)

   The whole "fish out of water" premise of the show tanked upon Rob Morrow's leaving the series.  Subsequently seemed to be left with no direction.  But it was to be the final season, anyway...

   

I'd agree about Roseanne and The Simpsons... I'll remember them for what was best about them, but they really started to stink after a while.

I kind of feel that Buffy the Vampire Slayer declined in seasons 4-7. The show remained interesting and intelligent overall, with a few of its best episodes in the later years, but the decline was still pretty noticeable. They did kind of "jump the shark" when they introduced Buffy's "sister," Dawn, in season 5. I have a hard time watching seasons 6 and 7 in particular, though, because Buffy herself becomes so self-absorbed and unlikeable. She develops the sort of tough, self-righteous "hero complex" that tends to make the main characters of fantasy series annoying... Though in the early seasons she's someone with whom you can identify, because even though she's "the Chosen One" she still tries to maintain a normal life as a teenager and she has real vulnerabilities. The high school years on Buffy (seasons 1-3) are definitely better, in my opinion.