When making an afghan or sweater, single crochet the seams together rather than using a slip stitch. That way, if it becomes torn or worn out, there is already enough yarn in the item for repairs. Just pull out the seam, fix the trouble and use a whipstitch to sew the seam back together.

You can also weave a long strand of matching yarn invisibly along each of several seams on the back side of the item, tacking the ends of each strand to secure in place. When yarn is needed to repair a tear, carefully snip the strand free at each end to remove and make the mend.

Both of these methods have the added advantage of using yarn that has worn or faded with the rest of the crocheted item so repairs are matched precisely and invisibly.