cache invalidation strategies, push versus pull?

Hey everyone, I've been thinking a lot about cache invalidation lately. We have several caching layers in our system, and keeping data consistent without bogging things down is a real challenge. Right now, we're using a mix of time-based expiration and some simple event-based invalidation. When it comes to clearing out caches, what do you prefer? Do you lean more towards a 'push' approach where the source directly tells the cache to invalidate, or a 'pull' method where the cache checks for updates itself or has a short time-to-live? What have you actually seen work well, and what hasn't?