It's also been in some other thread, maybe newbie questions at some point (probably, actually, considering the length of the thread)...
Anyway, besides editing the saves, i've read suggestions that you can grant your heir (this doesn't work in elective, for reasons that should be obvious soon) the title that you want to have as your primary, then wait for your current character to die and be inherited (or use the 'die' cheat, see sticky). Meanwhile, he'll be independent, but that's the price you pay.
The granted title will be his first count/duke/king title, so it will be his primary title, and that of his heirs from then on.
edit: My wish-list for CK2 would be some sort of function that gives you some choice on how you style yourself. Which title is primary, and for example the emperor trait could allow you to come up with a name for your empire, or maybe choose one from a list (HRE, the "emperor of all spain", etc.). Maybe some other traits could allow this too. This would differentiate ruling characters even further. Maybe also give you some choice on where your court is located, instead of the capital being chosen automatically (since the capital doesn't even have any game effect).