Techniques

The small size of the tree and the dwarfing of foliage result from pruning of both the leaves and the roots. Most trees require a dormancy period and do not grow roots or leaves at that time. Improper pruning can weaken or kill trees.
Copper or aluminium wire wrapped around branches and trunks holds the branches in place until they lignify (convert into wood), usually 6-9 months or one growing season. Some species do not lignify strongly, or are already too stiff/brittle to be shaped and are not conducive to wiring, in which case shaping is accomplished primarily through pruning.
Cultivators use deadwood features called jin and shari to simulate age and maturity in a bonsai. Jin is the term used when the bark from an entire branch is removed to create the impression of a snag of deadwood. Shari denotes stripping bark from areas of the trunk to simulate natural scarring from a broken limb or lightning strike.