Richard Marius' A Short Guide to Writing about History is a pretty good, practical guide to--as the name says--writing about history. What is most memorable for me from the book is that a good essay on history has to deal with some problem. If you don't have a problem, you don't have a paper. Another good point from the book is that it is best to address a problem that interests you or that you are curious about. If you are not driven to solve a problem out of your own curiosity, then you may feel less intrigued by the problem you are dealing with, or what you are writing about. Other advice includes the use of primary sources, how secondary sources can provide for details along the way to supplement your main argument and also for interpretation, and many other issues besides. It's a handy reference.