First, read - and immerse yourself in - the comment by /u/sunagainstgold. I always bow to the brilliance of the Sun.

That said, approach all sources - primary or secondary - with the understanding that everyone who has put pen to paper is trying to con you into believing something. Often, they are trying to con themselves. This requires a great deal of source criticism. In a few years, you may go to your ten-year reunion: that is an excellent display of people telling other people how wonderful their lives have become. Facebook is the same. And none of this necessarily has anything to do with reality.

Primary sources are a peculiar reflection of reality, and your task as a historian is to try to figure out what the author is saying about his/her time and how that portrait of that time may be affected by the personal point of view of the author. Then, as Sun points out, it's a good idea to ask why this source survived and others did not. Having just cleaned out my mother's home of over six decades, you'd be amazed at the primary sources I threw away - some mine and a lot from other people. They did not necessarily contain a portrait of the past that I would want some ghoul - I mean some historian - picking over in the future. A few documents survive. They portray me and my family as remarkable people.

Much of the same source criticism must be applied to the work of historians. We all say that we are attempting to provide a fair and balanced portrait of the past, but nothing is fair and balanced. The historian has a point of view that is generated by personal experience, training, prejudices - all sorts of things. I hope you wouldn't accept a discussion on TV of last week's events without understanding that everyone at the table is arguing from and for a specific point of view. None of them are objectively describing the events. They are all arguing. Your understanding of that current event emerges out of the argumentation, not by adopting the point of view of a single one of the voices. Unless you're one of those uncritical viewers who turn on a specific station, turn one's eyes to "glaze over" and open one's mouth to be force-fed the shit sandwich of the day.

The same goes for secondary sources. It's not always easy and don't be too hard on yourself. Your professor should be able to tell you what arcane debate your historians are engaged in - but they are engaged. They are fighting some battle, even if it is within themselves.

The most important thing here is that you should never accept the text on paper at face value. Whether written hundreds of years ago or thirty years ago about a time hundreds of years ago, your author has a point of view that affected what you are reading. You need to approach those words with skepticism and cynicism and take those words apart accordingly.

I read back and forth between primary sources and secondary sources. A couple articles or books or whatever to give me a foundation. Then I go dig at some of the meaty primary sources referenced. I've got some framework to start with, right, so that's what I'm looking at. From that perspective, something in the primary source will grab my attention. I'll read a little bit for more of that, then take whatever topic (&c) and look up some secondary reading on that.

This secondary reading can be related to the specific primary source at hand, or more general. Like, if I'm analyzing some letters by someone and get interested in how the letters were actually sent, maybe I go read a book about medieval communications even if it never once mentions Sophia of Admont.

This method is especially important for me because it's how I ensure that my sources tell me what my argument is, not the other way around. I don't have a thing to prove and then accidentally read my hopes/expectations into the sources. (Well, ideally. My advisor just busted me for that, kinda, on a little thing. I mean, I'm still right overall, but I needed to show it more carefully.)

Also, depending on what era you're working with, a lot of sources are a type or genre that you need to know the "rules" of, in order to understand. Like with my medieval letters example. You should know that often, the real message was conveyed orally by the courier; the written letter was more to provide a material connection to the sender. Sometimes, the content of the written letters are duplicated, even years later. Many medieval letters survive in letter collections, and you need to know that the letters were gathered for a purpose. To establish someone's sanctity? To be a sample of proper rhetorical prose? And of course, there's the question of forgeries, later copies of a letter that attach the name of a more famous person, "letters" that are really sermons, and so forth.

In medieval, we have a whole bunch of resources devoted to helping with this. I imagine classical history does as well, but it might be harder for other times and places where the source body is a lot more diverse. (So "reliability" isn't necessarily inherent to the source itself.)

Do those two strategies help at all?

/u/commiespaceinvader wrote an entry for "Monday Methods" that might shed some light. See also this entry by sunagainstgold, and a discussion of archives and the hunt for sources here by multiple flairs, most notably /u/Caffarelli. /u/Commustar hosted a series called "Finding and Understanding Sources" : (Part 1) (Part 2) (Part 3) (Part 4) (Part 5) (Part 6)

EDIT: Added the links to "Finding and Understanding Sources." I am particularly fond of #4.

The five W’s: Who wrote it? What for? Where were they? When did they write it? And WHY? Consider the political/social/economic contexts of the time and the background and agenda of the author. Sorry if this is a repeat - haven read the whole thread...