Fresh mint can be preserved by drying. If you have a dehydrator just wash and pat dry the leaves and place them in the dehydrator in single layers on the trays for about 24 hours. They will be crisp and dry and will last for about a year in a tightly sealed jar.
Without a dehydrator they can be spread on a window screen raised up above the ground in a sunny spot. Turn them over everyday until they are dry and crisp. Again they will last up to a year in a sealed jar.
They can also be frozen. Place in a single layer on a cookie sheet in the freezer. when frozen solid, place in a freezer bag and store in freezer until needed. By freezing them first on a cookie sheet you will be able to use just as much as you need at one time.
Hope this helps.