Bongs can get pretty gross. After prolonged use, resin starts to build up, and with the amount of mouths breathing through it, you can imagine what gets caught in its crevices. Cleaning a bong seems difficult at first glance, but a little alcohol, salt, and a good shake will loosen and remove any resin and gunk.
Take one-fourth a cup of both 99 percent isopropyl rubbing alcohol and rock salt (measurements vary for the size of your piece). Pour the solution into the bong, being sure to cover any openings, and shake until you see the resin begin to come off. Rinse and let dry before your next use. You can also clean a weed pipe with the same technique and solution in a Ziploc bag.