Here are some unrelated ideas for how to use categories:
If you are using Datebk mainly to keep track of business appointments and contacts, you might want to create several business-related categories (e.g., one for every company you are in frequent contact with) and very few (one or two) personal categories for your private matters.
Use categories as major dividers to group appointments and todos in big chunks. You can use icons to refine the classification. For instance, all birthday and anniversary events can be in a "Personal" category, with a "birthday cake" icon on to the birthdays and the anniversaries marked by a "valentine heart" icon. (See also BirthdaysAndAnniversaries.)
Create a category for appointments and todos which are very important. You can set the appointment and todo category profiles to use a large font and a blinding red colour; all items put in the categories will be given that font and colour by default.