It is a common observation that acrylic bongs get dirty quite fast. This is not really an issue if you are using glass since you simply need to rinse it using alcohol for a few times. Afterwards, the tar just melts away fast.
One thing to note is that acrylic and alcohol are not really good friends. The alternative? Salt and hot water. Make sure, however, in dealing with hot water because rapid temperature changes may result in cracks developing along the body of the bong.
What you can do is boil water, carefully pouring it into the bong, adding salt as much as possible. Doing so will help in scrubbing the inside part of the bong. Unlike sand or grit, the salt will not get stuck in the midst of the tar build-up and put the smoker at risk.
Afterward, shake it. This move will melt the tar, which means that it can become easier to remove later on. Thoroughly rinse the bong, slowly letting it cool down. Note that it is best to avoid rinsing it under cold, running water because it may cause cracks. You may also get a toilet brush that is designed to fit a bong tube, as it is also an effective way of cleaning it.