database sharding strategies, what works for you?

hey everyone, i'm running into a pretty common problem, but it's a tough one: how to scale up our main user database. we're basically maxing out what a single instance can handle, and i'm starting to think seriously about sharding. i've been reading about horizontal versus vertical sharding, and different ways to split things up, like using ranges or hashes for keys. so, for anyone who's actually done database sharding, what did you go with and what was the hardest part? also, any advice on moving data over without losing a ton of time or taking the whole thing down would be amazing.