You might think this one is obvious, but you’d be surprised how reluctant some people are to use a dictionary. Don’t expect your spellchecker to tell you the difference between “effect” and “affect” and where and when you should use one or the other. You should never assume you know what an unfamiliar word means, either.
For example, “tabling a motion” in British English means putting something forward (i.e. on the table for the Speaker to see it), whereas if you “table a motion” in American English, you are ruling it out, usually permanently. For this reason, it is probably better to avoid the word “table” used in this sense.
Dictionaries are also useful to ensure that the words you’re using really are words. For instance, we’re having an argument in this very office about the word “commoditize”, which doesn’t appear in dictionaries, but is now in widespread business use – and on Wikipedia. The Clarity jury is out on this one for the moment. In the meantime, our advice is to avoid any word that doesn’t appear in a dictionary.
If you don’t want to keep a weighty tome on your desk, www.dictionary.com provides all you’ll need along with an excellent thesaurus so you can expand your vocabulary beyond its usual limits.