This may not be what you're looking for, but the answer is just reading and writing more. It's the only way to improve. There are guides out there for analyzing texts and constructing arguments, such as Rampolla's Pocket Guide to Writing History. I would also recommend trying to find books that outline the frames of historiographical debates, for example Little and Rosenwein's Debating the Middle Ages. This book is meant to help undergraduate students understand the way scholars have discussed and understood a topic. Once you start to read scholarly materials, the authors will often simply assume readers understand the terms of the debates in which they are participating, going into specific only when they are making a contribution.

All that being said, these books are not nearly as useful as an instructor reading your work and providing feedback. I cannot stress enough the importance of having other people read your writing. Writing is hard, and when I was still teaching, it was rare for me to have an undergraduate in my class whose writing could be described "good". There are bad habits that simply detract from the overall quality of a piece, regardless of the novelty or quality of an argument. Feedback from someone who knows what they're doing is invaluable. In graduate school, my own writing and argumentation increased incrementally as I continually submitted work to my graduate advisor. The other thing that helps is the collected body of your peers who also provide feedback. Most graduate schools have workshops for particular subfields wherein students and professors read each other's works. These settings are invaluable for seeing other people's stumbling blocks so you can avoid them in the future, and, more crucially, expose you to literature of which you were not aware before. It's impossible to read everything, but you can look like a fool if you miss certain major works. The help and advice of colleagues is a tremendous boon.

In the end, my suggestion is to simply read history and enjoy it for its own sake. One does not need to speak and write like a professional historian to enjoy the subject.