Dan Pritchett has written an excellent blog detailing his experience with database sharding while working as an architect at eBay, Inc. and Rearden Commerce:
Lesson 1: Right Size Your Shards
Lesson 2: Use Math on Shard Counts
Lesson 3: Carefully Consider the Spread
Lesson 4: Plan for Exceeding Your Shards
Lesson 5: Shard Early and Often
You can read the details of each lesson in Dan’s blog.