Best way to mend a hacky sack?

Most hacky sacks are done in a crochet stitch. The most effective way to keep the shape is to join a strand of crochet cotton to the last secure stitch and crochet the hole by picking up all the secure stitches and single crocheting into all of them in rounds, decreasing each row as you get closer to the centre of the repair. If you just darn it, you will end up with a flat spot.