Wherever possible, avoid using repetitive words. Don't use the same word in the same sentence, or even in adjoining sentences, if there's an alternative.

Make friends with your thesaurus. If that doesn't help, try the dictionary. Even typing define:yourword into Google can often come up with an alternative for an unusual word.

And make sure you know what a word means before you use it.

Don't use a word just because it sounds right. If you can't give a dictionary-style definition to yourself, look it up. You might just save making yourself - and your employer/client - look ridiculous.

Syllables and Rhythm in Web Writing Using Headings in Web Writing

Related:
Using the Right Word
Dictionaries, Thesauri, and Usage Resources