The rain can be a wonderful ally in magick. It is intimately tied to the Moon because of the Moon's power over all water and because, due to atmospheric conditions, it tends to rain more so at Night than during the Day.
Rain is also associated with fertility and Mother and Father Deities; the rain brings life to the land and sustains it, the Father makes life possible and the Mother sustains it.
To effectively use rainwater in your magick, you must first collect it with intent. Depending upon how much you need, you may only need one jar. Hold it aloft, focusing on how you are going to put it to use, what it is you are gong to achieve. If quantities of rainwater are needed, you could set multiple jars down around in you (perhaps one at each Quarter) while holding aloft a larger jar that all of the water will be poured into.
Primary uses for rainwater are for cleansings and purifications, as the rain washes away all impurities. it can also be used for consecrating and blessing objects, as the rain is a sacred gift make life possible. Rain is also associated with mourning and tears and can be used as an aid to end the grieving process and get on with living again. And, as mentioned above, rain can be used for increasing or honoring one's fertility.
Throwing the Bones