Something to help loosen up phlegm and mucus?

i've had a cold for the past 3 weeks, and i dont seem to be getting any better. I think it is becuase i still have alot of phlegm and mucus in my sinus and chest cavity and i cant clear it all out. like if i try to hack it out i gag and swallow it, so i was wondering what i could take that would help loosen all that crap up.