I've recently decided to shift houses; and am simply amazed at just how easy it is now.
I found Share-Accomodation.net - just one of the many sites in this market space.
I really like the above - it's obviously thrown together by people without much English (typos abound); but the approach they have taken works like a charm.
You can post your ads and message for free - but you pay ($5 / 1 day membership or $20 / 1 month) to read your messages. By the time there's an inbox with several messages in it, it's much easier to justify spending the money.
It feels completely inoffensive, compared to the typical pay-before-you-can-message approach other sites take.
Highly recommended site