What's the best practice of Postgres sharding?

Hello,

We have one master/slave nodes running already, it works very well until now we need more business data to be stored.
But our running machine almost reached the read/write limit (1600 ops/s).

So what I am thinking is that I add more machines to store the new business data.But I don’t want to change my applications that query the data.
In short I want one single query entry, multiple machines to do data-persistence job.

What’s the best practice in this situation?

Thanks a lot,
James