when coming up with a new class what guidelines should you follow so that you are not making a class over specialized?
what i mean to ask is that the difference between a base class and a prestige class is how focused the prestige class is so how do you keep from making a single note 20 level long prestige class?
The first key is always concept. The concept needs to be broad and you have to be able to think of at least three to five different ways the class could be used (more is better). If you have an idea but you can only see it working in one way then your concept isn't broad enough.
I stress this again because of how important it is -- concept is key. The next part of the concept test is to make sure it's not something you can already do with an existing class, or that if you do it with an existing class it would be very unlikely to stand up to a full twenty levels -- also consider class combinations that could be used to fulfill your concept -- if it's something you can fandango your way to with two base classes and a few levels in three prestige classes it's again probably too specialized.
After this just build the 'chassis' of the class: BAB, HD, Skill points per level, skills, and good save throws and which spell casting schedule it will use (if any). Don't be afraid to steal abilities from other classes if they fit your concept, but also feel free to make your own.