On that site, they have access to DMPtool, which is a software package which will help craft a data management plan that will satify most grant requirements.
But if you still want to write your own:
Be as detailed and consise as possible. Include
1) a description of the project
2) description of how data will be collected
3) standards that will be applied to formats/metadata
4) plan for short-term storage and data management
5) legal and ethical issues of this research
6) access policies and procedures for dispersal of this data (such as an open access journal)
7) provisions for long-term archival and preservation