That's a crucial question. When you earn some income and you haven't yet reached what Social Security considers full retirement age, your payouts will typically be smaller during the period that you're working.
Specifically, if you start receiving your Social Security payouts early, but you're still working and earning more than $14,160 (the annual limit for 2010), your benefit is reduced by $1 for every $2 above that limit. However, Social Security will make an adjustment when you reach full retirement age to reflect the additional income you earned.
In the last year before reaching your full retirement age, the earnings limit rises to $36,120, and your benefits are reduced $1 for every $3 above that limit. As of the month you reach full retirement age, your benefits are not affected by any earnings limits.