As I mention below, lots of people ask me this question. Please post your answers here, but for now let me mention mine: A good English usage dictionary — my favorite is Webster’s Dictionary of English Usage, but Garner’s A Dictionary of Modern American Usage is good, too. (Garner also has A Dictionary of Modern Legal Usage, which may be worth reading as well, but it covers a different set of matters.)
Of course, this is also good advice for anyone who wants to go into a profession that requires writing English prose for a living. When words are your tools, you need to know them well, (1) so you can convey the right meaning, (2) so you can convey the right meaning without needlessly distracting or annoying the reader, and (3) so you can convey the right meaning without the reader’s concluding, fairly or unfairly, that you’re ill-educated.
And the trouble is that many people misuse words without knowing they’re misusing them, or use words that some dislike (again, whether or not the dislike is well-founded) without knowing that they’re risking condemnation. The dictionary is a good way of knowing where the linguistic land mines are.
Plus, at least to me, these dictionaries are fun reads, especially when you read a few pages at a time. Each page has some interesting and surprising tidbits, and the usage dictionaries are also pretty well-written; I’m particularly fond of the style of Webster’s.
Of course, knowing other things about writing — how to craft, organize, and edit sentences, paragraphs, and documents — is more important. But most of the people who ask me have already heard “read Strunk & White” (though I should note that not everyone is wild about it). They’ll have read it or something like it, though they probably will have a hard time putting it into practice; that’s just the nature of writing advice. In my experience, though, knowledge about usage rules is much easier to put into practice, even if it means just remembering that there’s a problem with some word (so that you can look up the word again). So that’s my tip; use it if you can, and suggest your own in this separate thread if you’d like.