(Sun 23:43 06/Feb/05)A good old fashioned way is to use white bread to clean the painting. Rub the bread over the painting. It should pick up any dirt and grime. The bread will absorb dirt and grime. After you've finished you can use a soft brush to get rid of any crumbs.
mrs.pudding
(Mon 12:57 07/Feb/05)A very weak solution of amonia diluted with water. Soak and squeeze out a piece of cotton wool and gently rub the painting. Keep changing the cotton wool and don't get the picture too wet. It can work wonders and shouldn't do any harm. I use about 1 part amonia to 5 parts WARM water.