Most of these techniques find their origin in Japan, where the art of bonsai got its modern day form. These are very useful in giving your bonsai a look of old age. Although your plant could be only 10 years old, but it will look like a much older plant.
This is technique which is used for recreating an image of natural damage and wear on a plant. Trees might have there branches broken during storms, or bigger branches might block the inner branches from receiving enough sunlight, making these branches die back.
Many bonsai artists take clue from nature, to create an impression of old age. To practice this on your plant, you need to carefully cut off a branch by making an incision into the branch using a branch cutter, and then use pliers to twist the branch of the plant. After removing the branch apply a good quality wood putty to seal of the exposed wood. This will protect the exposed part from fungal and insect attack.
In this technique you remove the bark from one side of the plant. This recreates the effect of bark die back which is usually found on old trees. To make a shari on your bonsai, first make an outline on the plant surface, clearly demarcating the area of the bark you want to cut. Then use a sharp edged knife to cut through the bark along the contours of the line you have already marked. After you have run through the outline, peel of the bark exposing live wood. Apply a good quality cut paste to seal the exposed parts.
The technique of promoting surface root formation, in a circular manner around the base of the plant is called as nebari. Many plants growing in the wild have such root padding, and this makes them look very mature and old, many bonsai artists use this technique to give a old age effect to their bonsai plants.
Many a times trees growing in the wild tend to have a hole in their trunk due to rot in the wood, or some other reasons, this make the trees look very old and mature. This is one of the bonsai techniques adopted by artists to recreate the impression of old age.
Another popular bonsai technique is carving. Use good quality tools like chisel and grinders and cutters to help you in carving out the bark from the surface of your tree.
Other bonsai techniques like creating splits and cuts on the plant, recreating an impression of root rot are also very popular amongst bonsai artists. To create the image of root rot, use a grinder to make a series of cut on the bark of the bonsai plant. Make the cut in downward motion of the grinder; this replicates the effect of root rot.