database sharding strategies for high-volume apps? dont you just love that question.

Hey everyone, my app is getting slammed with traffic and I need to scale it up. We're thinking about database sharding to spread out the load, but I'm not sure about the best way to do it. We've checked out horizontal versus vertical sharding, but I'm wondering about specific strategies, like sharding based on a hash, a range, or using a directory. For anyone who's actually done this for a busy system, what did you pick and what was your reasoning? What were the toughest parts about setting it up and keeping it running?