Here are some generic points:

 - Your topic has to be narrow enough, but it does not need to be as narrow as a specific research question. For example, the topic should not be as broad as "German politics" (too much literature to cover), it can be as broad as "policy change in German federalism", and it should not be as narrow as "the impact of the reunification on policy change in German federalism" (because that would be a specific research question).
 - At first, read as much on your well-delineated topic as you can. Once you recognize certain sub-topics, issues, questions, patters, debates etc., begin to organize your reading and further research around these emergent sub-topics. Decide, which you would like to cover in more detail. This adds structure to both your work and to your review.
 - Start to write from day one. At first, you will only be able to write short notes. Later, you will be able to arrange those notes around the sub-topics that you begin to discover. This is the outline of your first draft.
 - Your review needs a structure. It should answer one or a small number of questions. Resist the temptation to try to summarize everything that has been written on a topic. Instead, the purpose of your review is to chart the territory and identify the research frontier. Someone who has read your review should be able to identify the open questions and possible avenues to answer them in the future.

[More][2] along those lines.

And there is also [related question][1].


  [1]: http://academia.stackexchange.com/questions/3420/how-can-i-do-a-literature-review-efficiently?rq=1
  [2]: http://secondlanguage.blogspot.de/2009/08/reading-and-writing.html