Every person who has lived in the modern world has come across the great kitchen nemesis that is the stubborn jar lid; whether you're a cook dashing around trying to add the all important lobster base, or a guy who just needs that jar of jam open. I'm sure you've heard of some of the methods but some of the newer techniques may surprise you.
Things You Will Need
Kitchen sink, Hot water
The lids on containers are designed to fit with millimeter precision. A tiny bit too small and the lid doesn't fit, and a bit to big, the lid will not form a proper seal. It only takes a little bit of expansion to loosen a lid. Most lids on jars are metal and being metal, they expand a lot more than glass or plastic. Place the jar in the sink and turn the water on as hot as possible. It should only take 10 seconds to warm up the lid. Turn the water off and give the lid a twist. Another benefit of this technique is that it can sometimes soften the condiment around the lid that is acting as the "glue".
I learned this technique from a friend and didn't think it would work. It is incredibly useful in situations when you don't have access to hot water. Just grab the jar and a suitable surface like a cutting board. You want to smack the edge of the lid against the surface. You want to hit the entire circumference of the lid, so keep twisting the jar a little at a time between hits. I realized that the reason this works is because the hammering of the lid actually thins out the metal ever so slightly. Because you're squishing the metal and stretching the circumference of the lid a little, the lid has a looser grip on the container.
You can use a rubber band in conjunction with the other methods if you find your hand slipping or not having enough grip. Wrap a rubber band around the circumference of the lid.
Sometimes getting someone else to try is a good idea. Repeatedly applying pressure to the lid will slowly loosen whatever is holding it tight, even if to the naked eye, nothing has moved. And sometimes, they're just stronger.
Keeping jar lids clean will make it easier down the road. A sticky or dirty lid will undoubtedly get stuck. Hardware stores also sell special rubber twisting tools. These have a strong rubber belt that you adjust and a handle to unscrew pipes, handles, or bolts. These just happen to work incredibly well on jar lids, and I keep one in the kitchen.
Tips & Warnings
Make sure if you are using the smacking method that you do it on a surface that won't break or shatter.