One of the most common problems I see in my students' writing is their use of evidence. What constitutes good evidence? What is a good source?
The first thing I tell all of my students is that Wikipedia is never an acceptable source. Why not? Wikipedia is written and edited by a variety of people who may or may not have expertise in the topic about which they are writing. Wikipedia is littered with incorrect or dubious information, and should therefore never be cited in a formal essay.
A good, reliable source is one whose credibility can be verified. Books by known experts, articles published in peer-reviewed journals, and newspaper articles that rely on experts may all be cited in an history essay as proof of a thesis. Primary sources--interviews with historical actors, memoirs, photographs, artifacts--are also excellent sources of proof in a historical essay, but take care to provide adequate analysis of these. For example, citing the existence of a painting depicting George Washington next to a cherry tree is not in itself evidence that Washington chopped down the tree in question. What does the painting tell us? The answer to that question will allow a writer to use the painting as historical evidence.
Students should also take care to confirm an author's expertise. I hold a PhD in history, so a book that I have written on a historical topic could be considered a strong source. On the other hand, if I wrote a book on fashion, a student would have no reason to trust anything I've written on fashion, since I have no credentials to justify my expertise in this field.
What about perspective? Consider who is speaking when citing a quotation. If Dick Cheney is making a statement on a political issue, for example, students should consider his personal and political background when evaluating the statement. Cheney and, say, Elizabeth Warren can look at the same set of facts about poverty or war spending and reach entirely different conclusions about what the facts tell us. Remember to consider the source!