We do Words Their Way word sorts everyday. One of the biggest obstacles in implementing this was storing each student's set of 20+ words. Bags and envelopes just end up thrashed and words become floor confetti.
Enter the zipper pocket.
I found them at a back-to-school sale, but even at a discount they were $1 each. The cost almost scared me off, but they've proven to be worth 5 times that!
I've had this class set for several years and they still look really good. They take a lot of abuse and still come through. Yay for zipper pockets!