"If you can't duct it, forget it". I have to say that every time something involves duct tape because I am very mature and also hilarious. A lot of people are using duct tape wrenches to open jars, but I don't know why they don't just do a few more pushups or whatever.
Another trick is to put a knife or fork under the lid and break the airtight seal. However this only works if your stubborn jar is stuck because of the pressure seal. If it's rusted, corroded, or bent you might need to "duct it".