As a graduate student, a lot of my time is spent on solving problems, but not much time is devoted to the writing of mathematics. This is an important subject; especially if you wish for others to read your papers. While I am no expert, I have stumbled across a couple of good resources that are very helpful.
First, there is a talk by Serre titled How to Write Mathematics Badly. Jean-Pierre Serre is an accomplished French mathematician and Fields medal winner. This hour long talk is entertaining and very insightful. The video quality is poor, but definitely worth watching. If anyone knows of a better version of the video, please let me know.
Next, the book by Steven Krantz, A Primer of Mathematical Writing. While I don’t view this book as a canon for writing mathematics, I do feel it is a great guide and brings to light many issues graduate students may not be aware of.
Of course both resources focus on writing papers, but it would be good practice for beginning graduate students to use the techniques in writing up homework. For example, I believe Serre and Krantz agree that one should never begin a sentence with a symbol; it can be very confusing. Little things like this can help articulate ideas and make it easier for the grader to read.