During summer and fall, I enjoyed picking fresh shiso leaves from my community garden plot, and used them for Japanese cooking. But shiso leaves wilt quickly as soon as they are picked. Also shiso is annual plant, so that the leaves and buds must be picked, before the season is over.
To keep them fresh for a couple of days, wrap the stem ends with a wet paper towel and keep them in a ziploc bag to store in a fridge.
For long term preservation, there are 2 methods, depending on how many leaves you have. If you have shiso plants and harvested large amount of leaves at the end of the season, this method works well. If you bought shiso leaves from a store, and you can’t finish them all, try another method to preserve them.
When you use those preserved shiso leaves, soak them in water for 5 minutes to remove excess salt, then squeeze tightly or pat with paper towel. They can be chopped finely to mix in Japanese pickles, sprinkle over rice or add in Japanese flavored dressing. You can also spread the leaves carefully and wrap onigiri with them.