We've learned how old tennis balls can take scuffs off waxed floors, but here's another creative reuse: opening tightly sealed jars...
Cut an old tennis ball in half!
Then place the scooped out part around the lid, gripping the outer, fuzzy layer in your hand. The interior rubber grips the metal, providing a better seal.
photos: stock.xchng

Shaw's Original Fir...
Or...you can just wind a rubber band around the lid (preferably a nice wide one). Works (almost) every time!
Also, not sure how green it is, but a silicone hot pad works awesome. They are also great to put under a chopping board to keep it from sliding around.
Stick a metal skewer under the lid - breaks the vacuum, opens easily.
I just use a rubber dish washing glove.