This seems really self-evident, but only to those who have always done the decorating.
The first thing you need to do before you start decorating is ensure that the tree is stable. The last thing you want is to have a fully-loaded tree fall over, particularly on top of you. If at all possible, the tree should be away from the wall so that a person can get all the way around.
When decorating, it is a good idea to start with the lights. Before you even think about unwinding the strands, plug them in! It's better to find out which ones work before you wind them around a tree. Put non-working strands aside; there are methods for fixing them but just deal with the working strands to begin with. Run an extension cord to the tree. You might even choose to run the cord up the trunk and have the plug at the top. There's two types of serial plugs for Christmas lights; one is end-to-end and the other is plug-to-plug. Know which ones you have and plan before you string.
There's two methods of stringing lights. The first is the circle around the tree method; one starts at the top (or bottom) and winds the strands around the tree. End-to-end strands are ideal for this. The second is the branching method, a method best suited to plug-to-plug strands. Each branch is wound in lights starting from the trunk; on an artificial tree, careful planning can mean that the lights are stored on the branch itself.
The type of tree you have will determine the style of decorations you can put on the tree. Some trees have weak branches; they are best suited to lightweight ornaments. Some trees are open and can have ornaments all the way to the trunk. This type of tree is particularly handy when you have a surplus of ornaments. If you only have a few, however, you might want to go with a dense tree that you can't reach into.
Before decorating, you should consider the hazards your tree will go through. Rambunctious dogs? Perhaps the antique glass ornaments should stay in a box. Curious cats? Leave off the tinsel - it's irresistable and interferes with their digestion. Small children with sweet teeth? Restrict the candy canes to the upper branches alone.
The heaviest ornaments go closest to the trunk and the bottom as a practical consideration. On artificial trees, hang them in open spots to mask the metal trunk. One trick that is useful to know is that ornament hooks are bendable; crimp them onto the branch and the likelihood of them falling (or being knocked off) drops considerably. Light ornaments can go towards the top and the ends of the branches. Don't forget to decorate the back and sides; it is usually fairly obvious when the decorations are reserved to the front.
When all the decorations are in place, carefully move the tree back into position by kneeling and pushing from the base. (If the tree is light enough, lift it just a bit while someone steadies the top.) Plug in the extension cord and bask in the glow.
Next Up: Make Your Own Ornaments!