Disassemble them completely, including removing the ball lock posts (a 7/8" 12 point socket or closed end wrench will do the trick) and o-rings. Rinse thoroughly, and use some unscented detergent (OxyClean or Bar Keeper's Friend work well) and a long handled brush to scrub the insides down. Rinse and repeat until the inside is nice and shiny with no smell of soda remaining. Give the pickup tube a good rinse (it comes out as well) in the same detergent. Clean the fittings in the sink. If you like, you can scrub the outside of the keg with a mild abrasive cleaner (Bar Keeper's Friend) to shine it up a bit. Rinse everything VERY thoroughly. Reassemble with new o-rings and poppets.
The latest issue of BYO describes this in pretty good detail.