PHPMaster.com has posted part two of their series looking at indexing in MongoDB databases (part one here) with some more advanced concepts Mongo makes available.

In part 1 of this series we had an introduction to indexing in MongoDB. we saw how to create, use, and analyze queries with indexes giving us a good foundation to build on. In this part, we’ll take a look at a few more small but important concepts, like indexing on sub-documents and embedded fields, covered queries, and index direction.

They use the same "posts" collection from the previous article, showing you how to index it based on a "location" sub document and "embedded fields" inside of it. They also touch on complex sorting with a multi-field index and the idea of "covered queries." These are queries that all fields queried are part of an index as well as all returned. They finish up the article by showing you how to remove indexes too.