Bougainvilleas are perfect for training either way.
Note the highlighted area in the Barbara Karst image. The most critical part of creating a bougainvillea standard is forming a strong, straight stem. Line up the stake next to the stem, and push it down into the soil to the bottom of the pot. During the growing period, most people use twist-ties to secure the stem to the stake every few inches . For display, the twist-ties may be replaced with raffia or ribbons.