First, it's important to know when to use a quotation. Don't quote simply because you can't think of a better way to phrase something. Quotations should support your thesis, and they should come after you've already used your own words to explain something - not instead of your own words. And, when you do quote something, explain where the quotation is coming from; many students tend simply to throw in the quotation after their idea is stated. You have to "set up" a quotation.
Remember: Quotations don't speak for themselves. You need to tell the reader what the quotation means and how it relates to your own ideas.
Here's an example of a properly used quotation:
Many students at Skidmore have been concerned that they won't be able to find jobs after they graduate. One student even commented, "I'm worried I'll have to work at McDonald's for the rest of my life!"
Notice how the writer explains the concerns of the students before quoting. And notice how the quotation is set up by a lead-in phrase.
Here's an example of the same information with a misuse of quotation:
Many students at Skidmore have been concerned that they will "have to work at McDonald's for the rest of" their lives.
In the sentence above, the quotation is not introduced properly; the reader can have no idea who is being quoted. Is it a student? A parent? Someone from the Career Services office? Remember, your professors are looking for evidence that you can express your ideas and then support them with quotations, not simply use a quotation to replace your own words.