There is nothing that we can and cannot do, aside from deny Christ as savior that will earn us, or deny us heaven.
That would make US idols. That is puffed up pride to think that we have the power to save or condemn ourselves. That goes for man too...no other man has the power to forgive our sins, including priests. We don't need a mediator in Mary, a saint or man. If you have a relationship with him, just like your earthly parents, then there is no need to find a third party to talk to them and ask for forgiveness. You wouldn't ask a friend to talk to your father for you unless you are too astranged to speak to him on your own. The same goes for God.
Once we accept Him as our personal Savior and recognize that he died for our sins, it doesn't matter how we stumble, we are still saved.
However, the truth is in the heart. Someone who truly recognizes Christ as Savior is going to have a radical change in their heart and lives that will turn them from sin. The Spirit will gravitate you to things of God. You will no longer have a taste for the things that drive you to sin. That doesn't mean that you wont succumb to temptation, but that generally speaking, you won't be hanging out with the street gang, you wont continue dealing drugs, etc. It also means that someone who truly believes in their heart, will never deny Christ again. So the question of once saved, always saved never has to come into play. Someone who wasn't really sure to begin with will be the person who may turn to other faiths, or to satanism, wicca etc. They didn't become "unsaved", they were never truly saved to begin with. They heart and spirit didn't change therefore the heart and spirit wasnt involved in the decision to begin with.
Someone who has genuinely given their heart to Jesus will naturally want to please him and attempt to seek out the truth to change.