The first tip would be to check with your local university to see what resources they have available; many university libraries and English departments maintain writing workshops and assistance for undergrads and grad students. A good first step in writing better is to get more feedback on your own work, and try and be receptive and open to criticism, so that you can recognize where and why your writing is not being clear. For class assignments especially, consider forming a study group and exchanging papers for comment and feedback before the assignment is due. You might also consider auditing a technical writing course, which covers aspects of clear organization and rhetoric.

It's difficult to recommend specific books without knowing what your particular issues are with your writing; and there are many different styles of writing which may be applicable - academic writing for a grant proposal or assignment for class, for example, often leaves out elements of narrative storytelling which feature in history books and articles for popular consumption. If organization of your data is an issue, then outlining and similar techniques might be helpful; but if it's presentation of the data to support an idea, you'll need to focus on how you logically introduce an idea and examine the evidence.

A relatively decent overall guide to historical writing is Anthony Brundage's Going to the Sources: A Guide to Historical Research and Writing.

I sent you a PM, but if other people are looking for help getting the a good grasp for writing graduate-level work, Danna Agmon's got a great system for reading historical scholarship called THOMAS, and it is important to answer these questions when writing it, too.