How to do sharding:
1. Define your keyspace in bits.
2. Shard your keyspace in CIDR notation
3. Resharding always splits a keyspace in half
eg. 0.0.0.0/0 becomes 0.0.0.0/1 and 128.0.0.0/1
5. Assign IDs randomly
This gives a couple interesting properties, for replication odds are very high that your corresponding server has a shard of similar size and if not its generally only split over two servers. Servers also only need to speak directly to their one or two replicas in the other data center. The other really nice property if you're using spindle disks is that you can copy the entire HD and just delete the half of the keyspace not used after the split.
It's all written up here if you want to dig into with a full description of all the pieces: http://bit.ly/FredPatent