Today is the Ushpizin of Yosef Hatzadik, and tonight - Hoshana Rabba, is the Ushpizin of David Hamelech. The Zohar in Parshas Lech Lecha, discusses the merits that both Yosef and David had that enabled them to achieve kingdom.
The Pasuk states "And Yosef said to his brothers, come close to me, and they came close" Why did Yosef call them, were they not already close? But when Yosef said to them, "I am Yosef your brother" they were wondering, for they saw Yosef in the celestial kingship (through Ruach Hakodesh). And Yosef said to them, this kingship [that you see], because of "this" I earned it, come close to me, and they came close, and he showed them the sign of circumcision, and he said "this caused me the kingdom, because I guarded it". From here we learn that one who guards this sign of the circumcision, kingship is guarded for him. From where (else) can we know this? From Boaz, because he swore (when Ruth came to him alone at night) "By the life of G-d, lay here till the morning". Because his Yetzer was strong upon him until he made a Shavuah and guarded this Bris, and because of this, he merited that kings came out from him (David and his descendants), kings that ruled over all other kings and the Melech Hamoshiach who is called in the name of Hashem.
We also see from Boaz the power of vows. The Zohar says "his Yetzer was strong upon him until he made a Shavuah". That implies that once he made the vow, his Yetzer no longer bothered him even though Ruth was still there alone with him. The reason for this is because the urge to "act out" is psychologically tied directly to what one "perceives" as his "ability" to act out. However, if one is able to convince his mind that "acting out" is simply not an option, the entire urge simply vanishes. That is why vows are so powerful. For a G-d fearing Jew, once a vow has been made, it is simply "not an option" any more.
However, making vows is tricky and risky business. See this page for tips and advice on how to make vows in safe and effective ways.