I started reading Danny Wallace's Yes Man the other day (just by coincidence, it's been on my 'I should read that when I want something light to read' list for a while, and just out of a Neal Stephenson novel it was perfect).
In it, he says yes to all things, except if he's being asked a factual question and yes would be a lie (e.g. is your name Jemima). It's certainly a way to have a crazy year, but I wonder whether a less stringent yes regime might actually be a feasible way of catching some of the yes goodness without the bad stuff.
Personally, if I was to try a period of yes-ing, I'd add the following rules, to keep me sane:
It has to be an offer/opportunity/suggestion or something directed personally at me, e.g. advertisments don't count.
I don't have to do it if it would involve breaking the law (in a way that I would consider bad)
I would keep a budget for each month and would not be required to say yes if doing so would put me out of budget. I would save any money left each month to 'rollover' so that I'd have more at my disposal, in theory, each month.
I would not be required to say yes to a situation that would have a good chance of putting me in real personal danger (like accepting a lift off some random creepy stranger or something)
Or sexy stuff.
I would not have to say yes if it caused a clash with something that I had either said yes too, or had previously committed to. I assume if I have a commitment that means I'm doing something, and the whole point of the exercise is to make sure I'm doing things.
I wouldn't have to say yes to anyone who knew that I was saying yes to stuff, and was trying to exploit my yessness with their requests.
That's quite a big selection of exceptions, but still leaves a lot of things to say yes to. I'm not saying I'm doing that. I'm just saying that if I -did- do it, that's how I'd go about it.