Friction is often thought of as a bad thing: resistance to progress. But there's positive friction! For instance, record labels that only accept submissions in CD (or some other arcane) format. They're bound to get less of those than MP3s. A surer sign of quality? Not necessarily, but it's often interesting to find out who'll hop those hurdles and pass those tests. Who's really interested? You raise the bar, and the lazy and weak won't bother.
Laughing? Look at the dating scene. Witness self-degrading remarks like "She's out of my league". Heck, observe any scene where we test one another in hopes that we'll obtain the best. (Actually getting that is another story.)
Here's another one: building reputation online, or to put it less formally, playing an MMO and not just grinding stats, but establishing trusted friendships with many other players. The former you can sell or transfer, the latter you can't. Sometimes relationships between organizations work out that way, where Person A will only negotiate with Person B from another group due to prior history. Some call it "social currency", but unlike cash, you can't just give it away. However, you can empower others to rise up like you.
The positive friction trick, then, is coming up with a test that's hard enough to filter out and discourage the unworthy, yet easy enough to entice the incoming stuff you do want.
What's your method?