Lemon juice is a magical ingredient, in our humble opinion. It brightens a dish and makes its flavors distinct, but without actually making it taste lemony. If we didn't say anything, chances are that our guests would never know!
Other acids work well too. If you don't have lemon handy, try a few shakes of white vinegar or red wine vinegar. Lime is also nice in Mexican dishes, as in our quick chicken posole recipe.
The key to brightening a dish is to add the lemon juice or other acid slowly. Start off with just a teaspoon or two, taste, and then add more as needed. We usually find that there's a balancing point in the dish where the flavors suddenly pop out, but the dish can become too acidic if we go much beyond that.
Do you have a favorite method for perking up a blah-tasting dish?