Blot excess juice with a clean, damp cloth. Then squeeze lemon juice onto the stain, wait a couple of minutes, rinse, and let dry in the sun; the lemon's acid breaks up the stain, and sunlight has a bleaching effect. (Lemon juice works on cherry-juice-stained skin, too.)
For big, stubborn stains, soak the fabric in a solution of 1 quart warm water, 1 tablespoon white vinegar, and 1/2 teaspoon clear, mild dishwashing liquid for 15 minutes, then wash as directed. Make extra solution to store in a spray bottle for treating spots.