making reading faster, how to use caching effectively

hey everyone, thinking a lot about read performance lately, you know. we have this system, lots of data fetches over and over, and the database is starting to really feel it. i'm looking into different caching strategies, things like in-memory caches (redis, memcached, stuff like that) and also cdn caching for static stuff. but i'm really curious how other people handle this. what are your favorite ways to decide what to cache, how do you keep it up-to-date, and what tools do you like best? any tough caching problems you've managed to solve