When I bought my Lang smoker, Mr. Ben Lang walked me through a process that works pretty good for the larger smokers. He had me build a really hot fire, then once the temp was up in the smoke chamber, he hosed down the insides with just water. The water flashed to steam, and once it drained, the leftovers just wiped off with a rag. I pretty much do this whenever I see grease starting to build up. Real easy, no chemicals, and does not strip the oils that have been absorbed by the steel. Also, the hot smoker burns off any residual water to prevent rust.
Also, I have become a fan of using foil. I try to catch the grease before it makes it to the bottom of the smoker. This way there is no real cleanup, especially at competition when I'm really tired.