In addition to using prefixes and suffixes, you can increase your vocabulary by analyzing contexts and reading actively.

If a word is at first unfamiliar to you, look carefully at its context, paying attention to all the clues the context provides; often, you will be able to deduce the meaning. For instance, if the word accoutrements is unfamiliar in the sentence We stopped at a camping-supply store to pick up last-minute accoutrements, the context—a camping-supply store and last-minute—suggests strongly that equipment or some similar word fits the bill. And that is what accoutrements means.