Freezing a Sourdough Starter
Most people just freeze the starter like you would a container of tomato sauce--put it in a plastic container, seal and throw it in the freezer for a few months. You have to remember to thaw and feed at 3-month intervals. But you can also freeze-dry starters by spreading them in a thin layer on a large sheet of plastic wrap and sticking it in the freezer. The dry air of the freezer quickly removes the moisture from the thin liquid starter and you're left with freeze-dried starter that can be sent through the mail.
Because it's dry, the culture doesn't react when it reaches room temp again. To reactivate it, you need water. Now, mind you, that's the theory and I've heard people have done it successfully. There are variables, like how strong your culture is and how cold your freezer is (the colder, the better), so you might kill some cultures but it's worth a try if you want your friend to try mailing some from Japan.
Avital
My grandma taught me to keep starter from dough. She called it "making more yeast". She taught me to save about a golf-ball sized bit of bread dough, flatten it out into a thin disk, cover it with a towel and let it dry out, turning over frequently. To make more bread, you soften this in water and add a little flour and a pinch of sugar, then take it from there. I admit that I've never tried it, but I will someday.
Denise the sleepy.
Freezing a sourdough starter