To make sure all the dirt and grit are removed, try soaking greens; sand and dirt will sink to the bottom of the bowl.
A Good Soak
Drop greens into a bowl of cold water, and swish them around. Let sit for a few minutes, then scoop out with your hands. Repeat until water is clean and there's no grit left in bottom of bowl. It's even easier if you use a salad spinner; just lift out the basket to remove greens from water. To dry, use the spinner, or pat with a clean kitchen towel or paper towels.