Prepare your one-gallon Ziploc freezer bags first. Using a permanent pen like a black Sharpie, write AMISH FRIENDSHIP BREAD STARTER on the front, plus today's date.
Using an active Amish Friendship Bread starter that's been recently fed (past 24 hours), stir the stater well, and then remove 1 cup at a time (approximately 280 grams).
Pour the 1 cup of starter into a prepared one-gallon Ziploc freezer bag. Use a spatula to make sure you get a full cup of starter in the bag. NOTE: I usually pour in up to 1⅛ cup (1 cup + 2 tablespoons) as I know I'll lose some of it to the sides of the measuring cup or when I remove it from the bag when I'm ready to use it.
Shake the starter so it's at the bottom of the bag. Gently begin removing the air by slowly rolling the bag from the bottom. When you get to the top, seal the bag completely. You can use a rubber band to hold it together.
Pop it into your freezer. Done!
The starter won't freeze like ice--it'll be more like the consistency of a slushy. Once you put it in the freezer, you can leave it there until you're ready to use it. You don't need to take it out and mash the bag.
If you want to save a lot of starter, I still recommend freezing in one-cup portions. It makes it easier to gift, too, if someone calls to ask for some starter. You don't want to have to thaw an entire batch just to get a cup out.
Don't refreeze thawed starter. Use it, maintain it (start the 10-day cycle again) or discard.
How long can you save your starter in the freezer? Keeping in mind that I don't run a food lab here, I recommend freezing your starter for up to 9 months. It makes sense that your starter would degrade over time, so that's my conservative recommendation. Other factors include the quality of your bag (freezer quality vs something thinner), no damage to the bag (rips or tears), and a properly temperature-controlled freezer (max 0°F). That being said, I've revived frozen starter that was three years old and it came right back to life.