The need to consult a broad range of material has already been stated, but consider also the validity of the sources you review.
Part of your analysis of reviewed material will almost certainly involve a consideration of the theoretical underpinning of each source, inherent working assumptions, paradigmatic aims, and so on.
Explicitly articulate the rationale behind the theoretical aspect to your own findings and the position you have reached by the end of the literature review. Has later response in the literature provided damning critique of the work in question, or considerable support?
By formulating problems beforehand, you will avoid wasting hours in aimless reading.
Know the issues of concern to you and consider the material through this lens alone.
In any case, when considering your inclusion and exclusion criteria, it is important to ask the following of each article: is this relevant, suitable, and useful?
In practice, this process will ensure that the review remains concisely tailored to the topic discussed.
In order to write a satisfactory literature review you must demonstrate your ability to search out relevant material from a wide variety of sources.
Trawl online databases for useful dissertations and articles by using their abstracts to consider relevance, use all available university, college and departmental libraries, consult the web for extra resources, and follow footnotes.
If you're a new student or writing essays at a lower level where a literature review isn't a usual requirement, the concept of compiling one for a dissertation can be even more daunting.
It is a summary of the current writings in the field you are researching and into which your dissertation will eventually fit.