I have one usable "hanging" tree (a pecan) in my backyard. All the pines have too many widow makers for me trust them.
I tried using 2 4x4 post sunk 4 ft in the ground and back filled with concrete. They were set in a V with my one usable hanging tree at the center of the V and set about 15 feet from the tree and the post were far enough that I could hang 3 hammocks, one from each post to the tree and one from post to post. One post snaped while my wife and were hanging and the other one is getting ready to break. No one was hurt but we did get a good laugh.
While visiting a hammock store at an outlet store I saw they were using pier pilings/telephone poles to hang the display hammocks from. I stoped by my local marine hardware store and and was able to purchase 2 12 foot pier pilings (same piling they use to construct docks and piers with) for $45 each with free delivery. These pilings are about 7 inches thick and I will bury in the ground where the 4x4 post were.
The pilings will be deilvered in a few days (they said they would deliver free if I was not in a hurry).
While the hammock store I visited used screw in bolts with a hook on the end, I do not know if I would trust them. I found some closed eye bolts rated for 2,000 lbs that I may use as back up for the times if I don't use my adjustable webbing and Dutch Clips (all hail the mighty Dutch Clip).
After I get everything in place I will report back (with pictures as we all know if there are no pictures then it did not happen) and I can then get back to doing some serious hanging in my back yard.