Pansies have a very fine, fibrous root system. When you ease them out of their pots, the outside of the root ball may be a mat of white roots. Be sure to make several shallow cuts through these roots to encourage outward growth. It is also important to plant them at the same depth they were growing. They don't do well when planted any deeper.
Pansies are cool season annuals.
As the summer gets warm, their blooming slows down. You can also try cutting them back to within a few inches of the ground when it gets too hot and they may fill back out and begin blooming again in fall. Pansies also profit from regular feeding with a water-soluble fertilizer high in phosphorus. To keep pansies compact, be sure they receive strong light and stay well fertilized. This last few years, there seems to be a lot of pansies available in fall.