To plan values and value sets:
Choose a format for your values. See: Choosing Value Formats.
Decide whether your segment should have a list of values. See: Decide What Your User Needs.
Choose an appropriate validation type for your segment. See: Choosing a Validation Type for Your Value Set.
Consider using values that group neatly into ranges so that using range-based features (value security, value hierarchies, and so on) will be easier. See: Plan Values to Use Range Features.
Plan both values and descriptions as appropriate.
Plan any value hierarchies, cross-validation rules, value security rules, and so on as appropriate.