I wrote a handy FAQ on usability tests that can be reused if anyone is making a call for volunteers. Translate UX Participate FAQ: http://www.mediawiki.org/wiki/Translation_UX/Participate

For the translate UX improvements Pau and I are working on, considerable time was spent in managing communications and test schedules with volunteers. Because no volunteer had participated in such a test before, they often asked for clarifications on preparations they have to make and the duration. We now point them to this faq and it seems to have helped resolve common queries and make the tests something to look forward to, rather than a 'test'.

Feel free to extend this for future tests and pop some feedback for improvements.