database sharding strategies when and how

hey evryone im lookin to get into databse sharding for an app that s growin big. our single instance is hittin limits and we need to think abt horizontal scaling. whats ur fave sharding strat? are u doin range based, hash based, or somethin else? whats the biggest gotcha uve run into when implmentin sharding, esp around rebalancing and cross shard queries