Give it a few years. Sometimes, a later album released by a band you used to like will get better over time. I feel that way very strongly about 'By the Way' by RHCP, 'American Idiot' by Green Day, 'A Crow Left of the Murder' by Incubus, and 'It Won't Be Soon Before Long' by Maroon 5. Audioslave's last album was also their best, I think.
That said, yeah, some bands peak early and its all downhill from there. Arguably, Incubus' greatest album was their second, Maroon 5 and Linkin Park both peaked early and went downhill, RHCP's last album was a disasterpiece, as was Green Day's. That's not even starting with... [browses through library] ... Arctic Monkeys, Blink 182, Korn (best song ever was the first track of their first album), No Doubt (their latter stuff is literally unbearable), Staind, Tegan and Sara (first 3 or 4 albums are fantastic, last 2 are trash), Zebrahead, and, of course, Weezer.
Oh yeah, and my favourite sell-out? Liz Phair.