In no especial order of preference:
Chop and fill an ice cube tray, barely cover with water then freeze; FoodSaver or Ziploc.
Separate into individual cloves, FoodSaver or Ziploc; freeze.
Dice finely or mince. Saute in a little unsalted butter till just soft then remove from the heat. Allow to cool a bit in the pan (or cool more quickly in a bowl). Scrape into a Ziploc, lay the Ziploc flat on the counter and force out the air by completely flattening the shallot mix; seal and freeze. To use, break off a piece of the frozen shallot mix and put it directly into a heating saute pan--instant sauce beginning.