The short answer is that I don't know, but here's some thoughts.
It's unlikely that static electricity is the key. The different
procedures you name will change the static charges in very different
ways, so it's hard to see why all would work if static were important.
Perhaps little rough spots on the surface interfere with the
reading. Some of the procedures you mention could rub off bits of dirt
and others could wash them off. Here's a wild guess about how plastic
bags might work. These contain plasticizer- small molecules not well
stuck to the polymer. The plasticizer can leak out- a problem for
plastic food containers. It may serve to lubricate rough strips.
Here's what made me think of that. A type of copper high vacuum
seal was shipped in little zip-lock bags. One of my colleagues worried
that the copper might become coated with plasticizer, which could
evaporate, slightly contaminating the vacuum. So he went to great
lengths to clean the seals. Then they didn't work well at all. His
guess that they became coated with plasticizer in the bag was right. It
turned out, however, that the plasticizer was helping seal microscopic
leaks at rough spots.
I had a problem with a magnetic strip card I had left out in the
sun on my car's dashboard, and the plastic card warped and curled up on
the edges. Running this card through a strip reader didn't work because
the strip did not come in contact with the read head all the time.
Sometimes it help if I used my hands to uncurl it a bit (very difficult
to get it flat enough to read properly, and it proceeded to curl back
up over time). One thing that could be happening above is that if a
card is warped or bent, even just a tiny bit, it can lift off of the
read-write head when read. If the card is more flexible, then it can
bend back closer to its original shape when it is pulled through a
narrow slot. Warming up the card a bit will make it more flexible. Many
of the operations you describe above (especially rubbing it on a
sweater) will warm the card up just enough to make it more flexible.
I'm not sure how the plastic bag helps, but perhaps just handling the
card with warm hands will help.
(republished on 07/13/06)