For one I think the easiest way to avoid plagiarism is to cite all scources. It is very important that you give credit to the person/site where you got the information from. Secondly If you only use a couple sentences and dont feel that you need to cite the source, then make sure you re-phrase the words. Lastly choose familiar content. If you pick a topic that is very difficult and/or hard to understand, then you could wind up taking others' ideas and words. Even if it was an accident...this is plagiarism.