Please read a bonsai lifecycle if you have not already. This page gives more details to why the bonsai goes in the ground and what to do with it.
The fastest way to increase the caliper (width) of a tree trunk is to put that tree in the ground and alow it to grow unrestricted for some period of time.
1. Choose a plant for bonsai. This plant should already have good bonsai qualities. Feel free to browse my collection of stumps and unworked stock. However, one can even buy a pre-made bonsai and use this technique on that plant.
2. Plant the plant in the ground. Ensure the environmental conditions are appropriate for the plant. Make sure the plant is planted so that the roots are just below the surface - do not plant it too deep. Ensure the plant does not succumb to drought, it may require frequent water the first year while it establishes itself in the ground.
3. Leave the plant alone. Do not trim the branches. Do not dig around the plant. If you do not want to lose specific branches from the tree (for design purposes), trim these but leave others to grow.
4. When desired caliper is reached (at least 2 years), dig plant carefully and put it back into a pot. Cut off large branches that are now to large for the plant's bonsai scale.
Often these trees will still take years to become excellent bonsai.
