I am sure that this will be up for debate, but I feel the easiest way to clean you pipe or bong is to use Rubbing Alcohol, Salt and Rice.
The alcohol is what breaks down the sludge. You add the rice and salt to act as scrubbers. If you have a bong, You can add all these directly to the tube and shake away. Please try to cover any openings with your hand and fingers. If you have a smaller piece, just use an appropriately sized heavy duty ziplock for the agitation. Soaking also helps for the stuck on stuff. As always rinse well after doing this. Hot water really helps for the final clean. i also had a friend that like to use Simple Green as a final go through. He said it took out anything the alcohol might have left behind. I have seen alcohol leave a milky like residue. Simple Green took care of that.
The combination of all three of these will lead to a near perfect clean job. Some say that the rice is overkill, but I found that the rice and salt combo work in perfect harmony.
The cool thing about this cleaning method is that you probably have all of these components in your house already. If you don’t, you can find this at just about any grocery, drug or hardware store. No need to go to a headshop for any pricey solution. I am not knocking them, but they are not the only way to get a piece of your clean. Please be careful using this method on anything else but glass, ceramic, pyrex, or metal. I have heard of plastics and acrylics bust with the alcohol.