Place plastic planter inserts inside larger pots to minimize the volume of soil needed in the bigger pot and the associated weight -- this often is called double-potting. The inserts must have enough holes to ensure good drainage, yet they must not be allowed to stand in accumulated water inside the larger pot. Some gardeners use the original plastic pot the plant came in and just slip it inside a larger, more decorative pot.
Sometimes, gravel is placed under the insert to absorb excess water. The pot still needs checked and drained periodically to prevent any roots coming out of the plant liner from getting root rot, which is caused by excess moisture. Simply lift out the plastic liner, carefully drain the pot, and replace the liner.
Many potted plants require only 6 to 8 inches of soil for healthy roots. Large pots often have twice that depth, which encourages the roots, and eventually the plants, to outgrow their pots. It's also less expensive to fill the plant liner rather than the entire large pot with potting soil. The reduced weight makes the plants easier to handle.
The increased oxygen flow from below encourages larger, healthier plants and flowers. Plastic plant inserts also make it easier to change planting displays, to move plants into better sunlight or to put them indoors during cold snaps.