Here at E2, we are told not to copy and paste writeups and to give credit where credit is due by quoting statements and phrases which are not our own. I know that I sometimes have trouble paraphrasing because after I read something and then try to paraphrase it (by attempting to summarize what I have just read in my own words, without looking at the original), my "paraphrase" ends up looking an awful lot like the original.
An English teacher of mine from high school had a solution to this problem. After attempting to paraphrase, go back and compare your paraphrase with the original. If you used three or more consecutive words directly from the original source, quote them. I found this method very helpful in preventing unintentional plagarism.