cache invalidation a necessary evil tho

Caching speeds things up a lot, especially when you're reading a ton of data. But then there's cache invalidation, right? When the data gets updated, how do you make sure the old cache stuff is removed properly? What are your favorite ways or methods for dealing with that? I'm wondering about things like setting an expiration time versus invalidating based on events. What are the pros and cons you've actually run into?