The very best thing that I have found is a product called Stop-Flo.
It is an amazing product in that it will stop the solder from
flowing, yet is quite easy to remove one you have completed the
heating of the piece. You simply place the piece in water to soften
the Stop-Flo and it will allow you to take it off with nothing more
than a soft (used) toothbrush. The liquid has to be applied with a
brush and does dry quickly but it is well worth the price. It never
leaves a trace once you have removed it and it really is very easy to
remove, unlike yellow ochre or white out (that is a true disaster in
my book). It is available from the Mine Shaft in Ft. Lauderdale
954-917-7821. I am looking into making this one of the products to
carry, it is that good. One aside for the yellow ochre, you can mix
the powder with olive oil to make it more easy to remove than when
mixed with water. It still cannot shine a candle to the Stop-Flo
Beth Katz Paste and Powder Solder for Jewelers
http://www.myuniquesolutions.com