It is proper before one performs a Misva to recite the verse of "Vihi No’am Hashem Elokenu Alenu…" (Tehillim 90:17). Even if one does not recite the "Le’shem Yihud" text that many people recite before certain Misvot, he should at least recite the verse of "Vihi No’am."
The Ben Ish Hai (Rabbi Yosef Haim of Baghdad, 1833-1909) discusses this practice in Parashat Vayigash (6; listen to audio recording for precise citation), and notes that embedded within this verse are numerous profound, mystical concepts. He writes that although most people are unable to understand these concepts, let alone have them in mind while reciting this verse, there are two straightforward meanings that every person should think in his mind as he recites "Vihi No’am." Firstly, the Zohar explains this verse as a request that God should accept our Misvot and prayers as though we had all the deep intentions relevant to each Misva and prayer. Although we are not on the level where we can actually have all these intentions, we ask God through the recitation of this verse to consider it as though we perform the Misvot and recite our prayers with these intentions, and reward us as such.
Secondly, some Kabbalists interpret this verse as a prayer that God should restore the "light" of Kedusha (sanctity) that is removed as a result of our sins. When a person commits a sin, Heaven forbid, the sin has the effect of eliminating the "light" of Kedusha that he generates through his performance of Misvot. We therefore ask God to mercifully forgive our sins and restore this element of sanctity.
The Ben Ish Hai writes that one should have both these intentions as he recites the verse of "Vihi No’am." He adds that some have the practice of reciting this verse twice before performing a Misva, and concentrate on one of the aforementioned meanings during each recitation. This way, they do not have to think two different meanings during a single recitation. The Ben Ish Hai encourages this practice, noting that in any event there are Kabbalistic reasons for reciting this verse twice.
Summary: Before performing a Misva, one should recite the verse of "Vihi No’am," and have in mind two meanings: that God should look upon our Misva performance and prayers as though we had all the deep intentions, and that He should restore the "light" of Kedusha that we have lost as a result of our sins. Some have the practice of reciting the verse twice, each time with a different intention.
