I have never seen a glider whilst flying an airplane (of course, that doesn't mean that they're not out there, just that I haven't spotted them). But I have seen quite a few airplanes whilst flying gliders. Almost without exception, they have been far below me.
My advice for glider avoidance is to fly as low as is reasonably safe. Generally speaking, if you are less than 3000' agl you won't run into much glider traffic.
MLS-12D
P.S. I appreciate that this advice may be impractical for Boeings and Airbuses, but I don't consider them 'real airplanes' anyway!
