An extensive effort to explain the art of mathematical writing is the book Mathematical Writing by Knuth, Larrabee, and Roberts. The compressed plain TeX file of the text can be downloaded here. The book will be placed on reserve (if possible).
Another book on mathematical writing is Writing Mathematics Well by Leonard Gillman. I haven't seen this, but it has received good reviews.
For some examples of good undergraduate mathematics writing, see the Rose-Hulman Undergraduate Math Journal (click on "Downloads" or "Download Articles").
Here is some good general advice about giving math talksby Curtis Bennett and Frank Sottile
The internet is full of useful information on mathematical writing. A Google search on "mathematical writing" (in quotes) had over 13,000 hits, though I don't recommend reading all of them. Three nice sites are the following.