Guaifenesin is an excellent expectorant, it is extracted from the Guaii plant, and is very effective for thinning and loosening mucus.
Marshmallow will help loosen mucus also, but not quite as effective as guaifenesin.
I think vitacost.com sells it at a pretty reasonable price.
If the mucus is caused by a virus. Brew some pine needle and marshmallow tea.