"A votive offering is an offering made in consequence to a vow." (Thomas, 2007) Usually, a votive offering falls into category of sacrifice that delivers a service in a "client-patron" relationship. In general, this usually involves a basic formula of "if you do X, then I will do Y. . ." The aim of this request is to establish (or rely on) a relationship built on mutual gifts before you've given your first gift. Instead of making the first physical offering, the supplicant offers a promise of a physical offering in exchange for the desired blessing.
This sort of offering is one of the most provable through archeology: often, the vow appears to have been a promise to erect an altar if the Powers should deliver: as an example, at Domburg seafarers erected altars indicating the fulfillment of a vow to the goddess Nehalennia, likely for not drowning. (An altar to her reads Deae Nehalenniae; Dacinus, Liffionis filius, v(otum) s(olvit) l(ibens) m(erito), "To the Goddess Nehalennia, Dacinus son of Liffio freely and deservedly fulfils his vow.") (Espérandieu, 6644)
Fairy tales tend to offer gruesome alternatives to altars: first-born children are common votive offerings to evil spirits, as are beautiful and unwed daughters. A good reason to use a votive offering in a modern context, though, is the case of seeking a job through prayer: often, a person may be too broke to purchase an offering prior to obtaining the job, and so may promise to make the offering after the job (and the money) are secured.
A votive offering, however, requires follow-through. To make one and not hold up your end of the bargain is similar to "crying wolf;" eventually, the god you call out to won't believe that you are actually interested in the relationship promised by the offering and will withhold blessings as a result.
A Votive Prayer
Lugh of the Long Arm
Long have I laboured.
I come today in prayer
An offering of beer in hand.
I make this offering now to you
And call out to you as the skilled god:
[pour out beer]
As this beer pours forth,
know that I will bring more still:
If you shower your blessings upon me,
brightening my skill and showing my strength
in this interview today,
then I will bring forth a keg of beer
and will offer it all to you.
So is my vow, so may it be.
Lugh, I honour you.
