Deep condition with a color-enhancing shampoo. There are inexpensive formulas that gently deposit a tiny bit of color onto your locks each time you lather up. The difference will be subtle to you, and virtually undetectable to anyone else. They'll just keep your color looking bright and shiny longer. Choose a shade that matches your natural haircolor; if you have highlights, go with a shampoo that closely resembles your natural shade.
Suds up with a color-safe shampoo, if you don't feel comfortable using a color-depositing shampoo. There are many brands at different price points, from drugstore to salon lines, that hydrate dry hair without stripping it of color. Use these every time you shampoo to help prevent fading.
Protect your locks from the sun. The sun's rays can fade and even alter your pro dye job. Spritz on a UV-protecting spray any time you'll be spending a significant amount of time outdoors. Definitely for days at the beach, but even if you're heading to the park for an hour or two. If your hair is really delicate -- for example, if you went from brunette to platinum blonde -- consider covering it up with a head scarf or wrap to keep it from fading and drying out in the sun.
Use a glaze. A glaze locks color in your strands post-shampoo. Salons often apply them to freshly colored hair to seal in the color. You can keep the color trapped in by using a glaze in your own shower. There are colored glazes to spice up your hue, or you can use a clear glaze. Either version will enhance shine and keep your shade full of life.