The main barriers to listening comprehension (and I say this as a person with plenty of said barriers, so I know! ) are twofold: lack of vocabulary and speed/accent. Lack of vocabulary causes far more problems than the other.
You probably feel you have memorized a lot of Chinese words, and indeed you have. Unfortunately, memorizing them, producing them for a test, or even recognizing them in written form is a far cry from being able to immediately, INSTANTLY know the meaning when you hear them. Your brain has to stop and “decode” words that it is not 100% familar with – that means less brain resources available to continue listening to the next word of the sentence, and the next and the next – and if you hit 2 or 3 unknown words, or not-completely-known words – crash.
So now you have a twofold problem: increase your 'active-passive" vocabulary (we might call it “activated” vocabulary – stuff that you may have known before, but which is not as familiar as it needs to be) and increase your overall vocabulary (because there will still be many, many things you just haven’t come across yet, and these will trip you up, too.)
I always thought I was a purely visual learner, but I’ve been experimenting with more aural methods lately, and I’ve been pretty pleased with the results. One of my favorite exercises these days is to take an article in Chinese (get something not too scary as to level! and hopefully somewhat interesting), read it closely with any dictionaries or other aids I need, and then have somebody record it for me. Yes, it is written style instead of oralized (although you could have a talented friend make the style of the read passage more natural, like speech, while taking care to include the terms you have underlined or highlighted, to make sure you get hte repetition on them) but the main thing is that you get hte repetitions of those new (or recycled) words.
Another thing is to get out of the “school mindset” in which each item is of equal importance. They aren’t. Focus on the items that you suspect will be more frequently used or of more use to you (i.e., things that have to do with favorite topics, topics you need for work/school/whatever, and so forth). Don’t waste your time on fancy stuff until you feel more comfortable with the “basics” (if there are “basics” in Chinese ). Lots of people will be saying, “Oh, learn these chengyu” and so on, but I really think it is more valuable to you to learn what would be considered a core vocabulary. I have never seen a reliable list of this type for Chinese (I am talking about words, not single characters) for spoken language, but there are a number of wordlists for “Basic English” on the Net which make for interesting parallel references. Imagine that you could use and understand all the Chinese equivalents for those words instantly – you would probably understand a great deal more of what was said. And the more you listen to what is said, the easier it will be for you to produce accurate utterances yourself.
If you have an MD player, make yourself some “audio flashcards” – record the Chinese word, Englihs word and Chinese word again and make each of these units an individual track on the MD. THen set your player on “shuffle” and go do something useful while listening (work out, walk around, or something). It’s like flashcards, only for listening. This IS at the single-word level, so it could be argued that it is not as valuable as hearing the items in context, but then again the order is truly random, so that has its merits too. I use both these methods (the listening to passages and the audio flashcards) together and I feel pretty satisfied about the results (tested #1 in our class this semester including the into-Chinese portions of the test, so something must be working. I just hope it keeps working until well after the professional exam next year!)
Hope this helps.