I've spent most of today marking undergraduate student assignments - and I heavily penalised a couple of students for doing the opposite of what some propose here . always giving credit to your sources. Those students cited virtually every sentence in their assignment (what I refer them to as 'academic description'). There is a 'danger' in that advice though in that some students may not take the feedback correctly . 'don't always cite and write more naturally and flowingly' to mean I will not identify my sources so often. It can be a fine line with some.
Ask students for documentation. If a student’s work raises suspicions, talk with him or her about your concerns. Ask students to show you their in-process work (such as sources, summaries, and drafts) and walk you through their research process, describing how it led to the production of their draft. If they are unable to do this, discuss with them the consequences of plagiarism described in your syllabus (and, perhaps, by your institution). If you have talked with a student and want to pursue your own investigation of his or her work, turn to sources that the student is likely to have used and look for evidence of replication.