I had the same problem at a really great swimming hole near where I live. The tree (the only one usable) is massive, literally 10 feet around, with no branches. It is impossible to climb. The first branch is an honest 60-70 feet up. The method I used was I stapled (with a conctruction stapler) twine to a baseball. I then threw the baseball over the branch. This took many tries. I then tied a piece of lightweight rope to the twine, and pulled this over the branch. At this point I tied a heavier rope to the lightweight rope, and pulled it over. Finally I attached the real rope to the slightly heavier rope, pulled it over the branch, and hand-over-handed up this rope to get up into the tree to tie the knot and get it perfectly situated. A lot of work, yes, but well worth it.
For one reason or another I had to go back up into the tree several other times, when this rope was not up. Rather than re-doing my initial setup, I used the twine and small rope to pull up an old climbing rope. I then used technical rock climbing equipment to ascend the rope. This includes a harness, ascenders (jumars), daisy chains, atriers, various carabiners, and an ATC to rappel afterward.
It all depends on your actual tree. There are of course much easier solutions, such as the slipknot or quick link idea described above. I would recommend a quick link instead of a slip knot. These are available at any hardware store. Make certain the rope you are using for the rope swing is static, not dynamic.
