"Horolene" is a special solution available from some clock shops that is specially designed for cleaning the brass on them. It is often recommended for cleaning brassware. Unfortunately, like ammonia and a lot of cleaning solutions, it can leave the parts looking like copper (particularly the older brass items) if they are left soaking for too long. Dip, brush with a toothbrush and repeat if necessary is probably the safest method. Always dry thoroughly, for example with a hair dryer.  Horolene is available at: http://www.m-p.co.uk

 

You could try paraffin/kerosene, particularly if the parts are covered with grease or oil.  Personally I get perfectly good results even on greasy/oily brasswork with ordinary household ammonia and a toothbrush.

 

If you want to get rid of the tarnish off brass, I have found even brass polishes do not give as good a result as "Astonish". There are now many similar products to "Astonish" available though.  What you are looking for is a cleaner that is a thick white paste in a small plastic tub. It is made mainly of silica flour (a very fine abrasive - Just like polish), and the main cleaning ingredient seems to be sodium silicate. The active ingredient in dishwashing powders.

 I do know not to use vinegar, or presumably any other weak acid, as while it brings them up a treat, it seems to leach out the copper, meaning that even if well dried off, they develop a layer of green/blue verdigris fairly quickly.

Note that Horolene can be made:
Dissolve a teaspoonful of green soap and a teaspoonful of crystallized oxalic acid in a coffeecupful of hot water.
Now comes the outdoor part.  Add a little concentrated ammonia - just enough to "make it smell."
Ten minutes in the bath is ENOUGH - if you wait too long, there is a risk that your couplings etc. will be "cuprified."
Rinse in methylated spirit.  Note that oxalic acid is poisonous. 

Also go into The Smithsonian site on cleaning antiques…ammonia and liquid soap.