Every time I change blades on either of my mowers, I just get another one down where they hang in my shed. Both mowers take the same blade and I have six of them (the two originals and four more). After changing it, I take the dull one and clean, sharpen and balance it. Then I paint it with gloss black caliper paint and hang it up. I've been doing this ever since I got the second mower in 2008. At this point I think the blades and mowers might just outlast me. :)

Oh, the caliper paint was just because I had some that I bought for something else. The stuff is really good paint.