While not recommended on a windy day, when you’re down on places to prop your bike, look for a curb:
That’s not a magic trick.
The key is proper use of the pedals:
The bike is resting solely on the pedal, so it’s tough to get this right. Use the weight of the bike and the position in the pedal stroke to hold the bike up… Looking at the crank, I’ve got the pedal at about 6:30 – 7:00. If you were to set the pedal at 4:30-5:00 this wouldn’t work as the freewheel would simply allow the crank to spin backwards.
I’ve also seen a helmet used in lieu of a curb in a pinch.
Just do yourself a favor and practice this before you use it. This isn’t a very stable way to stand your bike up – and it’s always better to opt for leaning it against something. In a pinch, though, this will do.