...make sure the recipe allows enough acid. Whether that means adding lemon juice or vinegar, the pH should be 4.6 or lower.
...allow for proper headspace (distance between the top of the jar and its contents): half an inch for pickles and a quarter-inch for jams, butters, etc.
...take care to be clean. Jars must be sterile, the work space must be scrupulously clean, and the lip of the jar where the lid sits must also be immaculate. Any food caught between the jar and the lid will prevent a good seal.
...allow the right amount of time for processing according to the size of the jar and the altitude. If using the "hot fill method," make sure what goes into the jars is between 180 and 190 degrees.
...be intimidated. Just follow the few safety guidelines and you'll do fine. If the first result is not perfect, just pop it in the fridge and eat it up.
...pour hot liquid into a cold jar or place a jar that has cooled too much into a water bath. It will break, and that's no fun.
...forget to check the seal. The lid should be taut once the jar has cooled. I usually allow mine to sit overnight to cool completely before storing.
...forget what Mom said when you were young: People like something you made for them more than something you bought. So give your canned goods to friends and loved ones.