This section is from the book "Preserving And Pickling", by Mary M. Wright. Also available from Amazon: Preserving and Pickling: Two Hundred Recipes for Preserves, Jellies, Jams, Marmalades, Pickles, Relishes and Other Good Things.
Complete sterilization of the glasses and jars, and also the utensils used is necessary if your fruit is to keep perfectly. This is done by immersing the glasses and jars in hot water, or better still, placing them in cold water in a vessel and bringing the water to the boiling point. A little baking soda added to the water will aid in this sterilization. Keep the glasses and jars filled with boiling water or immersed in boiling water until you are ready to fill them. Don't forget that what is necessary for the jars is also necessary for the lids.
Always fill the glasses or jars to overflowing, and place on the lids while hot. The rubbers should be dipped in boiling water before using. When screw top glasses or jars are not used cover the top of preserves, jellies, jams and marmalades with melted paraffin before putting on the lids. In these cases do not fill the glasses quite to the top, but allow room for the paraffin.
Cans should be perfect, and rubbers new. Nearly all the failures in preserving and pickling come either from incomplete sterilization of utensils, careless handling, or an insufficient amount of heat, or because the jars are not sealed properly.