How do I clean kale and other leafy vegetables for cooking?
When washing vegetables that can get muddy (Kale, leeks, etc.) I generally recommend first using tepid to slightly warm water to help soften the mud more easily.  Leeks should be split down the middle and then can be fanned under warm water which will help wash the dirt out more readily.

For Kale, if it isn't muddy then you can use cold water and give it a soak and agitate as hobodave indicated below.  If they're muddy and dirty, then I'd use slightly warmer water to first clean them and then give them a soak in some cold water to help plump them up and increase their turgor pressure to make them nice and crisp.

A good wash in water (universal solvent) should be enough.  Most of what I've read on the effectiveness of "vegetable wash sprays" say that they're no better than a good wash with water.