there are numerous ways.
i smoke the resin because you can get igh off of it and it seems like a waste to just throw it away when it still has use.
what i do is take 90% rubbing alchohol (or something higher) and pour some in my pieces (you should make sure your bubbler is completely dry before you do this) and plug all the holes with my fingers and shake it for a while, pour out the liquid onto a glass plate and light it on fire so that the alchohol burns up, then i repeat again and again until my piece is completely clean and there is a good amount of resin on the plate, then i let the water evaporate.
i then scrape the resin off with a razor or something.
if you want to get that alchohol taste out of your piece blow smoke through it (cigarette smoke works great if you smoke cigarettes) if you dont smoke tobacco then hit a different piece and then blow the smoke through it.
after your done with everything your piece should be clean and not smelling of alchohol.