It is a very useful lost art. I learned how to do it in Virginia when I was in my teens and have done it on occasion ever since. Depending on the plane any wind over 10mph will work but to really have fun it needs to be near 20. I have built one ship dedicated to wind flying. It had a Nobler wing and tail with a slab fuselage and no motor. It took two to get it in the air but once airborne you could fly as long as you wanted. Lazy 8's are the main way to build up speed but once you get it moving and learn how to keep the speed up you can do most anything as long as you kept it downwind. Short lines and yes I have flown alone using a U-Realy hooked to a Flight Streak Jr. with a busted Fox 15 for nose weight. Easiest way though is to get some old POS engine and put a tiny tank on it to get you in the air.
The launch is critical. You have to find someone dumb enough to run with the plane over his head for about 1/4 lap starting up wind and throw it just before downwind and you have to be ready to whip it like crazy as soon as he lets go. Just as the nose starts to turn into the wind you can start into a lazy 8. A couple of eights and you will have found the optimal turn point (which is well before you get dead into the wind) and you are good to fly until your mother says "Dinner is ready". I have personally stayed up over 30 minutes.
Ken
Ken's been there, done that. I've never done it with a U-Reely, but that might be the easiest way to start. My wind flying was all done when the engine quit after a powered launch. On 60-foot lines. And the airplane makes a LOT of difference.
I couldn't justify building a special model for it and used mostly combat models. One was the Sterling T-Square; it had replaceable motor mounts and mine needed several replacements. The airplane literally fell out of the air at some point - repeatedly. I didn't mind much because mine was covered with Silron (silk-rayon blend) and was sturdy as a brick.
My favorite was the "Nadir" - a takeoff on Don Still's NObody. At 18 ounces on close to 500 square inches of wing and a thick airfoil it didn't take much wind to keep that one up pretty much as long as you wanted.
The "trick", of course, is to keep the airspeed up. Judicial use of all whipping techniques is appropriate. And yes, lazy eights are the obvious first move - but not a limitation by any means. Just ask yourself what you can do in half of a circle when you have airspeed.
I have an especially fond memory from an AAA combat contest here about a hundred years ago (1959). My opponent's pit crew stepped on his streamer when launching at the beginning of a match; the judges immediately alerted us we would be rematched.
When his engine quit, the judges hollered out we'd be restarted after my engine had about 5 minutes to cool off. So, when my engine quit, I decided to wind fly that 5 or so minutes away.
In the rematch, my opponent never started his engine. He fiddled with his prop, removed the glow plug, toyed with his battery … To this day, it must be said I won that match by wind flying!
That's my story, and I'm sticking to it.
PS: The (Riley Wooten) Quicker in the center of the photo below is mine. When we flew on a snow-covered field on cold Minnesota winter days, you can bet that airplane did a lot of wind flying before I put it down 'cuz the sound of a hot engine landing in cold snow is outright ugly!