A little while ago, I found this thread over at CastBoolits:
http://castboolits.gunloads.com/showthread.php?83572-Citric-acid-brass-cleaner&highlight=citric+acid

I tried it out last night and it worked very well!  A very light pink tarnish on some of the cases, but they were shiny and clean inside and out.  This is a good solutions for some of us apartment dwellers who don't have a tumbler.

Instructions:
***DO NOT USE A METALLIC CONTAINER***
***I have not tried this yet on nickle plated brass or studied what it will do***
-1 teaspoon citric acid concentrate / pint (16 oz) hot water
-2-3 drops of dish soap detergent
-shake/stir, let soak for 2-10 minutes
-rinse thoroughly
-dry

I will take some pictures later, but I am satisfied with the results.  You can pick up citric acid from your grocery store for around $3, it is often found in the canning section.

I am leery of using chemicals to clean brass because can attack the brass on a molecular level.    Ammonia being one of the chemicals that will affect the brass...   Just beware, I posted some info awhile back about the embrittlement of the brass metal.

The citric acid method was endorsed by the NRA many years ago.  It does not etch the brass.  In fact, it will return some of the tarnish back to brass.

This cleaning also applies a layer of passivation to the brass, meaning it will be slightly harder on the surface, and will be more corrosion resistant in the future.

The_Shadow is right about knowing exactly how your brass will react to a given set of chemicals though.  Any ammonia or ammonia seed chemicals (chemicals that decay into ammonias) will be very bad mojo for your brass.

Shadow, while you are correct about the dangers of ammonia, citric acid does not have similar deleterious effects with yellow brass (cartridge brass).  Ammonia does significantly weaken brass and should not be used near brass cases.  For this reason, Windex should not be used near cases.

Citric acid does not actually etch or dissolve the brass, it works as a reducing agent.  It works only on the surface of the brass, taking the oxidized layer (corrosion) and reduces it back to brass.  Theoretically you could leave the brass indefinitely in the solution and it would not be harmed.  It will also protect the brass from future corrosion.

Additionally, I just did some more research and it appears that this wash should also be safe with Nickel cases.  Citric acid reacts very strongly with iron, but not with nickel or chromium.  This is why citric acid is often used to passivate stainless steel to protect against corrosion.

Hope this makes sense.

sqlbullet, sorry I was typing my reply while you posted yours!  I should have read yours before posting.  Sorry for the duplication!

Its cool, the citrus acid product works like you are reporting then that's great.  I was just making the statement as a warning as many polishing products also contain the ammonia base...

I use a vibrating tumbler and before I place my brass, I put some charcoal lighter fluid (teaspoon or two)(my reducing agent)  on the corn cob media and let it run to even out without clumps, then add the brass!

There is a decent thread about wet tumbling with SS and citric acid on ar15.com. Probably a few threads actually. I have tried the method where you just let it sit in the water, and with SS wet tumbling. The tumbling works a lot better.  I only run new (after fired) for about 30 mins and that is probably longer than needed.

The citric acid really is helpful when you are trying to clean up old black stuff. I have some stuff from the 60's I found and it looks like I bought it new.