Data Sharding: How do you pick your shard key?

hey folks, been thinkin a lot about data sharding lately. were scaling up and need to split our main user database. the big question for us is how to pick the right shard key. weve considered user id, but that might lead to hot spots if new users sign up in bursts. any insights or war stories on choosing effective shard keys and avoiding those dreaded hot spots? what strategies have worked for you