How can you keep yourself chaste?

Luther:

That is certainly not easy. According to Augustine it is the most difficult of all the battles against sin. All saints have complained about it and have had grief about it, as Paul says in Romans 7:18: ‘For I know that in me, that is, in my flesh, dwelleth no good thing’. You have to work hard and with much effort to restrain your lust. Much eating, drinking, sleaping, hanging about and goofing off are weapons of unchastity and they easily overcome chastity. On the other hand, Paul calls fasting, being awake and using godly weapons with which you can restrain unchastity (Romans 13:12). You can only go so far with those to fight and dampen unchastity, but you cannot suppress natural desire itself. Further, the strongest weapon is prayer and God’s Word. When lust stires, you have to fly to God in prayer and call upon God’s grace, read the Gospel, and dwell on the suffering of Christ.