This is more an art than a science. The [Mongo Documentation on Schemas][1] is a good reference, but here are some things to consider:
- Put as much in as possible
The joy of a Document database is that it eliminates lots of Joins. Your first instinct should be to place as much in a single document as you can. Because MongoDB documents have structure, and because you can efficiently query within that structure there is no immediate need to normalize data like you would in SQL. In particular any data that is not useful apart from its parent document should be part of the same document.
- Separate data that can be referred to from multiple places into its own collection.
This is not so much a "storage space" issue as it is a "data consistency" issue. If many records will refer to the same data it is more efficient and less error prone to update a single record and keep references to it in other places.