Once they are over 6 years old they will no longer appear on your credit file. Most lenders will look at when a CCJ is registered rather than if or when it's repaid. The logic is that the time of registration is when you had financial difficulties - so as mandub says - the older the better, as this indicates that it has been longer since you had your difficulties.
Simple things like having current credit commitments that you are paying on time and ensuring that you are on the voters roll will improve your credit rating.