"I notice you have a lot of vacancies. Is there any chance to get a room (cabin, suite, etc.) at a reduced rate?" Or "I notice you have a lot of vacancies. Can I get a room at X% of your regular rate?" and leave room to negotiate. If s/he says no, then say "Ok, thanks anyway, I'll call the next one on my list" and move on to the next one.

When antiquing you can usually get 10 to 20% off of the ticketed price if you ask nicely.

Be willing to walk away. If possible, make the other person say a price first.

Questions like 'What's the lowest price you could accept for this whatsit?' will make you feel like a used-car salesperson, and rightfully so. To be a successful negotiator, though, you need to be able to put those feelings aside.

If you're willing and able to pay cash, be sure to mention that.

My thinking is they'd rather have someone there paying SOME money than to have the place vacant.

I've learned the hard way that with vacation houses, this thinking is often wrong. There tends to be a collusion amongst owners not to have the perceived value of their property drop, and not to take risks by ending up with a different demographic in residence than the one they'd envisioned. Sometimes they actually would rather not rent it at all than change the price structure - especially because they are likely already enjoying a good return on this investment, and the off-season takings are gravy.

However, I have had luck tacking on extra days for no additional charge, or asking for a round number that's a bit less than the full per night price, prorated, if I stay through an otherwise unsold weekday.

It never hurts to ask.