Reorienting Thoughts Towards Virtuous Object

I noticed that if I set an intention to avoid some behaviour, I rarely follow through. I feel perhaps a mere intention based on willpower doesn't have much strength. **Is it more effective to direct one's thoughts to a virtuous object whenever fantasies and cravings, or worries and rumination, occur? Are these cravings/worries causing the problems?** Is *just* setting an intention mentally sometimes not adequate?