Dawn dish liquid, a scrub brush, and the hose. Suck out the rinse water with a shop-vac. If you want them to smell nice, clean again with Resolve carpet cleaner. It's recommended you remove the seat pan and back cushion from the rails before hitting it with the hose. I've done j-yard seats smeared with axle grease using this method. They come up nice.
Edit: Please don't take the hose to your leather seats...
Edit: Please don't take the hose to your leather seats...