When canning fruit jams and jellies that are notorious for turning out runny, like blackberries or peaches, add the peel of an apple to the mix while it is boiling with the sugar, then remove it before filling the canning jars. The pectin found in apple peels will help to stiffen up the jelly without having to add extra sugar or expensive store bought pectin. (This tip only works for cooked jams, not freezer jams.)
