Scientists believe that the common ancestor of chimpanzees and humans slept in the trees, but it's been awhile -- some six million years [source: University of Cambridge]. So, you'll need to brush up on your climbing technique and purchase some special equipment before you go tree camping.
The first thing you'll need to do is choose a good tree in which to camp. Find one with plenty of open spaces between the branches. For maximum stargazing, climb a hardwood in the winter when leaf-off will give you a clear view of the sky. Just make sure the tree isn't bare of leaves because it's dead; a rotting branch could mean a premature end to your tree camping adventure. Of course, to sleep high in a tree you'll need to climb it first. This task requires specialized equipment like rope, pulleys, throw lines and ascenders, as well as a healthy knowledge of knots like the Blake's hitch, Prusik knot and figure eight. Be sure to find an experienced tree-climbing veteran to teach you the skills needed to use these tools and techniques (and read How Tree Climbing Works).
Once in the tree, climbers have two main options for sleeping arrangements. One is called a tree boat, a lightweight, rectangular hammock designed so that a rope can be attached at each corner and tied to different branches, typically in the same tree. These four anchor points make the bed exceptionally stable -- you can even stand up in it -- and more importantly, they prevent you from rolling out at night. Another option for tree sleeping is a portaledge, a flat nylon platform supported by a rectangular aluminum frame. These can be disassembled, carried up a tree by a climber and reassembled, or simply put together on the ground and hoisted up in the tree where it will be ready when the climber gets up to it. Whichever option you choose, try to set it up during daylight when you can see better and are therefore less likely to make mistakes.
Tucked away high in the branches of a tree, you'll need to be prepared for a number of scenarios. Bring a mosquito net and rainfly in case unwelcomed bugs or inclement weather disturbs your snoozing. If low temperatures threaten, pack an inflatable camp pad to help insulate your backside from the cold. Perhaps nature's greatest inconvenience to tree campers, however, is when it calls. Since climbing down in the dark to use the bathroom isn't a particularly safe option, you'll need a container in which you can, well, you know.
Whether you're setting up, sleeping, or using your makeshift bathroom, remember to always stay tied into your safety harness. Tree climbers call these saddles, and they're similar to the harnesses that rock climbers wear. Ideally, you should tie the saddle to a tree branch that's not also supporting your hammock. This will help to prevent a nasty fall, even if a branch supporting your bed were to break. And it will keep you safe in the unfortunate event that you unconsciously decide to do some sleepwalking!