As the plant grows, snip off any new leaves that appear on the lower stem and add more ties to the top growth to protect it from wind damage.
When the plant reaches the top of the stake (or when it reaches your desired height), snip off the tip of the central stem. This will force new growth to appear below that cut and eventually give the plant a rounder, fuller appearance. As the plant continues to grow, trim the tips of the side stems to shape the plant. Set it in a protected area away from strong winds and heavy rain.
Tips On More Plants
Coleus aren't the only summertime plants that can be trimmed into topiaries. The following three plants also make interesting standards using the same techniques.