Be direct with people who are gossiping about you or others. Tell them that what they're doing is hurtful and that you refuse to participate in such discussions.
Change the subject. If your boss, a coworker or a close friend is gossiping and you feel uncomfortable asking them to stop, change the subject so that you don't have to participate in gossip that may end up hurting someone else's feelings.
Prevent gossip by paying attention to what you do and say around others, especially around those who tend to gossip. While it may not be fun to censor yourself, at least you won't be providing any new material for people to spread rumors about.
Tell your secrets only to trusted friends to be sure that no one is spreading your business around town.
Let it go. If you've already asked the gossipers to stop blabbing about you or others and they're still at it, move on and let it go. The gossipers will eventually get bored and move on, and you'll be happy that you didn't spend any more of your time worrying about what they had to say.