How do you handle database sharding when you've got huge amounts of data? What are the best ways to do it?

my system is getting close to petabyte size, and our current single big database is really struggling. we're starting to think about sharding, but honestly, it seems super complicated. what are your favorite ways to set up database sharding when you've got tons and tons of data. are there any particular ways or tricks that worked well or were a pain in the neck when you actually used them?