Glass bongs can be cleaned easily with pipe cleaners and Acetone. The easiest source of Acetone is most nail polish removers! Residues left from smoking are very difficult to remove with washing up liquid and many other cleaning products i've tried, but Acetone is a solvent for the residue.
Be sure to replace the metal gauze that goes in the burning section. This stops ash falling into the water reservoir thus helping to keep the bong clean. Gauze can be bought from shops such as Affleck's Palacein Manchester, UK.
If you have a glass bong, make sure the joints are greased before use, this protects them against scratching, and also provides an air tight seal.
