database sharding strategies: what works for you? dont you think its time to figure this out

hey all im lookin to shard our main relational db as we continue to grow and scale were hittin some read/write limits that are becomin noticeable ive been readin up on diff approaches like horizontal vs vertical sharding keybased sharding and directorybased sharding for those whove implemented db sharding what strategies have u found most effective and why what were the biggest challenges u faced durin implementation and ongoing maintenance