In the art of bonsai wiring is one of your most powerful tools to control the shape of your tree. The best time to wire a tree is in the spring or fall when there is not too much foliage, and the tree will not be too stiff to bend from being dormant. (The sap pressure in the trunk and branches in much lower in the winter in most species of tree)

The basic idea of wiring is to wrap the truck... and then each branch in a spiral of bonsai wire so that you may then bend the branch using your fingers. (Rather like those little bendy dolls.) The tree will then train the branch to grow in the desired direction. Another method of wiring involves attaching weights to the branches, causing them to sag and creating the impression of age.

Generally, the wire is left on for one growing season. You should not allow the tree to outgrow the wire, since this could cause the bark to become bound to the wire and making removal traumatic for the tree. When the time comes to remove the wire, cut it away in little pieces (rather than winding it off) since this will cause less damage to the foliage.

The thickness of the wire you use should match the size of the branch. Larger branches will require higher gauge wire. If you don't have thick enough wire simply use two pieces of a lower gauged wire paired together. It's considered bad form to let any of your wires cross. If you start from the trunk and work up this should be fairly simple.

When bending the branches listen and feel for any sign of splitting. When bending a branch near the truck use extra caution. Often the branch is most brittle near the trunk. It is possible to gradually bend a branch over the course of months, further and further-- never bend a branch too far at once since you may destroy it. Essential to the art of Bonsai is great patience.

As you work with the branches consider the style you want your tree to be in. Look at trees in nature and try to create branch shapes that mimic those of old beautiful trees. Do not make silly or abstract shapes. (I mean you can ,if you want to... but that's not Bonsai... that's more like topiary)

Also See:
Care instructions for the Juniper Bonsai
Bonsai Palm
Zen Rock Garden