Christians where do you find the fine line between legalism and freedom to do what you want?

So some Christians think once saved always saved. There are a lot of scriptures that support you can be a believer and even be filled with the Holy Spirit and still choose to walk away. And that you cannot intentionally live a life of sin if you want to follow Christ. And then there are those who are so legalistic and impose rules on themselves and other people that even Jesus wouldn't have and the Word says we cannot boast of earning our salvation...how do we find the balance and walk the narrow path? Any scriptures would be great! :)