Picking the Right Way to Invalidate a Cache

Hey everyone, I've been having a tough time with cache invalidation recently. You know, for some of our services, the data updates pretty often, and making sure the cache stays accurate is getting to be a real pain. We tried just setting a time-to-live (TTL) for things, but that either leaves us with old data or makes the cache work harder than it needs to. What methods have you guys found work best in different situations? When it's super important that the data is up-to-date, do you go with time-based, event-driven, or maybe some other way?