While 听不懂 is a useful phrase that any beginner needs to learn, it can't help in all situations. What someone is communicating with that 听不懂 is essentially, "I don't understand anything you are saying," when in fact it is only part of what is said that gives him trouble.
There's a solution to this problem. When you don't understand, you can communicate what you don't understand better. Sometimes the person talking is drunk, or old, or young, or suffering from a speech impediment, or mumbling. None of that is your fault, but you do have to deal with it.
Here are some options for when you're ready to go beyond 听不懂:
什么？我没听清楚。-What? I didn't hear clearly.
This one is good partly because it's not the 听不懂, which immediately clues the listener into the fact that you may, in fact, know more Chinese than just a handful of phrases from a phrasebook. Also, claiming that you didn't hear clearly kind of implies that if you had heard clearly, you may have understood. Give yourself a little credit. People frequently don't speak clearly.
我没明白你的意思。-I didn't understand what you mean.
Be clear, this is not the same as 听不懂. This sentence may be used by native speakers when they understood every word, but the sentence doesn't make sense to them or the speaker's meaning is unclear. So this one is perfect for those situations when you understood every word but don't know what the person means.
你在说谁？-Who are you talking about?
This one only makes sense if you're reasonably sure the person is talking about somebody, but you're not clear who. Obviously, this can really backfire if they weren't talking about any person, but most things people say involve some person, so there's a little room for error here.
你的意思是…… So you mean…
Sometimes your best bet is to just guess what the person means. Don't underestimate the usefulness of this strategy! I've seen beginners with 5% comprehension totally guess what a speaker means, while an intermediate learner stands next to them with 60% comprehension, dumbfounded. The difference is paying attention to context. One of the advantages of guessing the speaker's meaning is that you're kind of "showing your cards." You're giving the person an idea of your vocabulary and listening comprehension level. And sometimes the words you use are enough to help them modify what they said originally into a form you can understand.
There are a lot of others you could use too, and probably all of them are better than 听不懂. You just have to put yourself out there a little. Don't shut people out with your non-comprehension. They'll help you if you let them.