There's no hard and fast rule, but the rule of thumb is to start with your company's domain name in reverse:
com.mycompany
Then add on the project:
com.mycompany.project
This ensures you're unlikely to have clashes between your classes and those from the libraries you depend on.
Then personally I try break things down by their functional groups, for example
com.mycompany.project.domain // contains the business domain classes
com.mycompany.project.io // contains the classes that deal with network or file-system
com.mycompany.project.persistence // contains the classes that handle persistence of the business domain classes
com.mycompany.project.ui // contains the user interface related classes
Within those packages, I might have further group but that would be very specific to the project.
The important thing is to be consistent across your project.