The first thing to say on this score is : DON'T DO IT let me repeat that DON'T DO IT. This may be more difficult than you think but as long as you cite your sources and quote your quotes, fully, you should be OK.
Most schools will give out plagiarism guidelines, style guides etc. Read them, you'll get better marks and you'll avoid plagiarising by mistake.

This node is about avoiding being accused of plagiarism when you actually haven't done it. You may think this is pointless because if you haven't done it you will beat the accusation anyway. This is true but being accused of anything is not nice, at all, and is likely to distract you and cause problems between you and your accuser. My final project (dissertation) tutor (BSc) accused me of plagiarising an essay and we never really got on after that to the detriment of the project that was 40% of my degree. I would rather have avoided that.

Many of the following points amount to the same thing: consistency. Be consistent, there's nothing that arouses a professor's plagiarism sense than inconsistent work. It should also be noted that just doing one of the things listed below is unlikely to get you accused but a combination is much more suspicious. So here goes the checklist:

  • Quality Consistency: This works in both ways; if you submit a piece of work that is much better than your normal, or even much worse, this will arouse suspicion. That is not to say that you should do sub-standard work on purpose just for appearances but be aware that your professor may not be expecting greatness.

  • Style Consistency: If you normally write in broken english and then hand in a piece of beautifully written prose with all correct punctuation you are playing with fire.

  • Typesetting Consistency: If you always use Word and then suddenly switch to LaTex (then you may be learning something!) you may have problems. It should also be pointed out that a lot of the academic content on the internet was written with LaTex so when using it for the first time you will probably be subject to extra scrutiny. You should still use it though because you are likely to get better marks for style (but that's a different story).

  • Time Reasonableness: If you have left that piece of work to the last minute, don't let your tutor know. That way when you turn in a masterpiece your tutor won't be amazed and puzzled as to how you managed to do all that in 24 hours.

  • Relevance: Stay on topic! You are unlikely to be able to find an essay with the same title as the one you have been set on the internet. This means that if an essay is off topic or fails to answer the set question you're in trouble for so many different reasons. Not the least of which is that your tutor will probably assume that you did download it.

If you think you have done too many of these things then you'd better be prepared to discuss your work, sources and content in detail.
And finally, Don't let the bastards get you down and cite those sources.