Method 1: Get a conductive strap that bolts to the frame - when you
stop, the end contacts the ground and bleeds the charge off the car.
These work well, but wear out rather quickly.

Method 2: Keep your hand on the edge of the door (or any metal part)
as you exit the vehicle.

Method 3: Whenever you exit your vehicle, hold your key and make sure 
that the first thing you touch after you exit is made of metal, and 
that you touch it with your key first.  This will dissipate any static 
electricity.

Method 4: a quick spray of a product such as Static Guard also helps 
to eliminate the static buildup from rubbing on the cloth seats.
(Thanks to Jim Janecek <Janecek@Tezcat.com>)

Method 5: Drive naked. Then you won't build up any static electricity
between you and your clothes. If you wet your pants, that should
dissipate the excess charge as well. <g>

Also from Jim Janecek:

"You might want to also check the type of tires on vehicle, if they are 
'low rolling resistance' tires they may have more of a silicone base 
instead of a carbon base and this does not allow the static charge 
that normally builds up on a object moving through the air to 
disperse through the tires.  The silicone base is more of an insulator 
than the carbon base.

Unfortunately, I don't have a list of what tires have the silicone base 
and what have the usual carbon base in them.

I just know that Michelin had a series of 'low rolling resistance' tires 
that came as factory standard on some recent (2-3 year old) model 
Hondas and they would not allow the static buildup to bleed off into 
the ground through the tires, so when you stopped at a toll booth 
and touched the tollbooth operator, the operator would get a real 
big shock."