Keep your pressed flowers in a sealed container with a lid. It's important to use a container with a lid because leaving your pressed flowers out in the open air will cause them to fade and break down faster.
Store pressed flowers in a dark, cool location, away from direct sunlight. A closet would be a perfect location. If the flowers are exposed to too much sunlight, they will quickly fade and lose their color.
Keep your pressed flowers out of rooms with high humidity, such as a bathroom or kitchen. Humidity will break the flowers down quicker and they will also lose some of their color.
Purchase a flower dye kit from a craft store. Dye kits contain acrylic paints that you can brush onto your flowers to help preserve the color. Dyed flowers keep their color for several years.