Next time you apply for a job you can try to negotiate yourself a 4 day work week. Ask if they're willing to give you a day off in exchange for a 20% pay cut.
I've never worked more than four days a week for any employer other than myself (usually three, with four and five day weeks popping up occasionally during big projects). I've always made less money because of it, of course, but from my very first real job, I've always been officially a "part time" employee. All of them occasionally asked if I wanted to go full-time...and I always refused, though sometimes the offers were quite attractive.
I've missed a couple of jobs because of this insistence of mine of not working five day weeks, but I don't think any of them would have been a particularly good fit for me, anyway, so no big loss. If the employer isn't a really good one, other employees could be bothered by one person only working three days a week, though, so I can understand employers being hesitant to hire someone with that requirement. And, when I begin hiring, I suppose I'll have to think about these kinds of issues more seriously.
It doesn't work nearly as well if only one person gets a 4 day week. As he points out in the comments, it works at 37sig because everybody works the same days (M-Th).
