Hair lacking in luster? You might be suffering from build-up. Start by using a clarifying shampoo to remove stuck-on conditioners and styling products. After you wash and condition, try a quick cold-water rinse -- hair scientists won't tell you that this trick works, but many swear by it.
Conditioners with a little silicone can add shine, as can silicone serums; avoid leave-in conditioners. Spray-on shine products work well, but don't overdo it, as they build up quickly and can look greasy. For a serious case of the dulls, try a colorless gloss.
Another cause of dull hair is damage from processing or overbrushing. Damaged hair cuticles don't lie flat, so they can't reflect much light. For more on this problem, see the section on damaged/overprocessed hair. And if your hair is dull because it's dry, see the section on dry hair.
If your hair is curly, keep in mind that you'll never get a really high shine -- it's just not physically possible. Because it bends and twists, curly hair can't reflect as much light as straight hair.