HOT water, some Dawn dish detergent. Scrub it out with a bottle brush as suggested. When all the coolant and "easy" contamination is gone, you're ready for Round Two.
Round Two involves a "short fill" of HOT water and Dawn, but throw in some Playground Sand. A few nuts 'n' bolts, anything small and durable. Put the cap back on, pinch the hose shut. Shake the crap out of it. Let the sand scrub the inside of the tank. The nuts and bolts agitate the sand. The longer you shake it, the cleaner it gets.
Dump out all the sand and hardware, rinse well.