Isopropyl alcohol (rubbing alcohol) works wonders, especially if you get the 90% instead of the 70%. It's a solvent and will literally absorb the build-up. Basically fill a plastic baggie with rubbing alcohol, enough to submerse your piece & then let it sit & soak for 2 hours or so. The longer you let it soak, the cleaner it gets. Then just grab some ear swabs or pipe cleaners and get the rest out.

I heard an oxyclean solution is an awesome alternative as well.

If you don't clean your bong or pipe very often, bacteria and mold can easily grow and make you sick. It will make your smoke taste like shit.