Pre-existing condition coverage for private individual insurance plans is going to be pretty standard. 12 months where they won't cover it or 6 months if it's treatment-free for that long. If it's a very serious condition, such as diabetes, that might be uninsurable under an individual health plan or very very expensive.
I have heard about some health plans specializing in pre-existing conditions, I think usually state-sponsored plans, but I don't know much about them and they very from state to state. What state do you live in? Maybe someone knows more about the plans available in your particular area and could point you in the right direction.
Also, what is your pre-existing condition and how much on-going treatment do you need for it?