Writing
First, read. Go to the library and get a book. Or, if you're on the way home, buy a well-written magazine like The Economist or the New Yorker. Read it and pay attention to how the sentences and paragraphs are put together.
If you're writing a paper, you may want to look at my research tips and suggestions
You'll also want to browse Temple University Library Catalogue & Electronic Resources
Mechanics
Start a file on your computer in which you keep interesting, odd, or just plain difficult words and their definitions. This will help if, god forbid, you do the GRE. Expanding your vocabulary isn't a bad idea.