Couple o'thoughts from a guy who has been around a while and learned a few lessons about airplane tie-downs.
Everyone is focused on the knots at the wing tie-down ring, and I use the double locking half hitch described above, minimum of two, three ain't bad ... the other end of the line is tied to the anchor in the earth. The bowline knot is absolutely the best way to secure your line to the tie-down in the ground. No matter how tight the line gets in a wind, the knot is easy to un-tie when the time comes.
Another idea, if you know your airplane will be subjected to a wind event is to zip-tie the tag line to the taut line (the photo below says it all). That way, the wind doesn't toss the tag line around and loosen the last knot. Just be sure to take something with you to cut the zip-tie the next time you go to un-tie your aircraft.
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Linked Image]
Yes, that's water in the background ... mine is a float plane, and it too needs to be tied down.
I have one other tip to add if I can find my photos, or take new ones.