I use a special solution made up of two parts hydrogen to one part oxygen. It's generally thought to be benign to humans, but in very large quantities it has been linked to numerous human deaths, so caution is justified. I find it especially helpful to use the solution while it is in a relatively excited state, so if you can find a way to add energy to the compound, flux removal will be hastened.
If you're unable to find a source for the compound, hot tap water may work too.
HTH!