Earlier quoted context omitted.
Depends a lot on your audience. In our case we were making a general consumer product, so we just advertised on Craigslist saying we were doing market research. You get a lot of dross that way, but my cofounder had a sophisticated screener questionnaire and ways to root out most of the goofs.
I've found CL hit or miss as well. It's good to hear that screening can work. I also pay a "stipend."
Two screener tricks that I recall:
One is getting in a little market research. Add a few questions that you've been wanting to know anyhow.
The other is including plausible-sounding answers that let you reject liars. For example, we had a grid section that asked, "How often do you use these sites?" One axis was frequency, "several times a day", "daily", a few times a week", etc. The other axis was the name of a bunch of sites, including popular things like Facebook and Amazon. But we'd also make sure to include more obscure things, and a couple of entirely fake ones.