“It strikes me that mathematical writing is similar to using a language. To be understood you have to follow some grammatical rules. However, in our case, nobody has taken the trouble of writing down the grammar; we get it as a baby does from parents, by imitation of others. Some mathematicians have a good ear; some not (and some prefer the slangy expressions such as “iff”). That’s life.” -- JP Serre
Check out this page for information specific to math, including several videos illustrating how writing works in a math class.
Kevin Houston's video on writing in math -- Part I -- and here's Part II.
Kevin Lee from the University of California at Davis has written this handbook, A Guide to Writing Mathematics
Guidelines for Good Mathematical Writing -- and even mathematical writing can be elegant!
Ten Tips for Writing Mathematical Proofs
A blog on writing math
Wendy Hoffer's chapter on writing like a mathematician. (Note: this is about to be published so you're getting a special preview of her work.)
Using Writing to Deepen Student Learning
Michigan's Department of Education has prepared a document about writing to learn in math.
Steven Strogatz's columns on math
Jason Brown writes columns on math
How about having students do some fun writing about math, something similar to Bedtime Math or Sir Cumference and the Adventure of Pi?
Interesting article for the formula 1/e: How to Marry the Right Person: A Mathematical Solution
What's up with the mathematics of beauty
Sports statistics reveal the importance of math