A couple of times I've read a book in a series and didn't really love it, but I also didn't
hate it enough to completely give up on the series itself. Then I buy the next book and stare at it on my shelf for about a year or two before either reading it or giving up and getting rid of it. So.
Luckily I haven't run into too many series that got crappier and crappier as the books went on. Mostly I think that's because I either have forgotten to continue reading the series altogether (which happens more often than not, actually) and so I miss out on the crappy.
Lately I've been trying to NOT buy the next book in a series if I wasn't wowed with the last, and that's actually been working out pretty well. I just keep reminding myself that if I can think about the series and don't feel an urge to read the next book, then it's okay if I don't force myself to do so. I have too many other
good books to read to waste time on a crappy series! And that seems to work. Sort of.