You may improve this statement by saying: 1. My key to future success is discipline.** 2. The key to success in the future is discipline. 3. My key to a successful future is discipline.** 4. The key to a successful future is discipline. **On the other hand, "key" here sounds a little awkward with the possessive "my" (of mine). It is more common to say, in English, "the key." If you specifically want to talk about your personal success, perhaps you can say: "The key to my success in the future is discipline." OR "Discipline is the key to my success in the future." These seem like the best options. I guess you could say "Discipline is my key to a successful future" as well. Although, you are saying "MY key," it still sounds more natural than the others. To answer your question regarding "of," it is always best to eliminate unnecessary words when communicating. Eliminating the use of this word is one way to be more concise. Example: success of mine> my success friends of Michael> Michael's friends future of success> successful future Source: American English (self)