I think that, though counterintuitive, one of the most effective ways was to limit the number of questions a user can answer per day. A casual user will still answer the 11th question the next day, and more hard-core users will try to answer as many questions as allowed. For this to work, the limit has to be pretty small, at least starting out, so that it is reasonably easy for someone with other commitments to achieve it. Setting a small limit also requires users to choose which questions they most want to answer, which increases the likelihood that their answers will be useful. When more answers are useful, it increases the usefulness of the service, which in turn "incentivizes" people to answer questions. And by placing a premium on the number of questions a user can answer per unit time, the likelihood that the user will throw him/herself into it full-throttle but then quickly become bored is also decreased.See my answer to the question, "Why am I only allowed to answer 10 questions a day here?":http://answers.yahoo.com/question/?qid=1005120801767On the issue of the term "incentivize"--it is shameless corporate-speak, but it describes an impersonal, profit-oriented process. To attempt to use softer language to describe something that is inherently cold, calculating, and opportunistic (though not necessarily bad or wrong) would constitute euphemism, and would be both misleading and dangerous.