I use a standard rock climbers harness. Usually costs about $60 for the more comfortable (sport) versions and it has leg loops that help support your weight when hanging. I also use a loop of static climbing rope wrapped around the tree and clipped with a 'beaner to harness. Two ropes give you continual security. Any good outdoor shop should have some and be able to show you how to use it properly and some even have rope you can hang by to see how comfortable it is to you.

The problem with the Prussic knot is that you have to get that main rope up in the tree first.So the first ascent it doesn't help you any.