Just fill your sink with water and then give those greens a few good swishes. Leafy greens will actually float on the top of the water, so dirt sinks to the bottom of the sink while the greens stay on top. We usually let the greens sit in the water for a few minutes so the grit has a chance to settle.
If you have a smaller amount of greens, you can also do this in a large bowl - or right in your salad spinner. We just find the sink to be handier when we're cleaning a whole load of greens at once.
We actually first heard this tip on an episode of Good Eats with Alton Brown - just one of those completely life-changing tips he sneaks into his segments. Thanks, Alton!
(Image: Emma Christensen)