My vocabulary is quite limited, and as a consequence, my prose is rather terse, and I'm writing in english twice as slow as in french. (I'm usually much more verbose in french.) To remedy to this, I began to read english (or american) literature (Under the Volcano to start. Next books on my list : Thomas Pynchon, Herman Melville and Philip K. Dick, my favorite american writers.)
gnat's notes on writing made me think about one thing or two. For example, I'll never mistake its and it's : they look totally different to me. I don't know whether this is caused by the fact that I learned the english grammar before I learned to speak english. On the other hand, I never mistake a and à in french (that's a similar grammatical error).
While I've a good understanding of the general rules of english grammar, I'm unfamiliar with the good practice of written or spoken english. That's why I'm reading english books now. My english often looks like french -- to me anyway. That's weird because I tend to think in english for technical matters. (Similarly, my grandfather, a Sicilian immigrant to Tunisia and then to France, was using french for some subjects, sicilian for some others.)
A final remark. I've always thought that really good programmers are necessarily good at grammar (and that people that are very good at grammar are potentially great programmers). My experience has generally confirmed this. I think that understanding the grammar of a natural language and learning the grammar to interact with a computer involve roughly the same brain wires. I wonder what the opinion of Noam Chomsky would be on this.