There are companies that manufacture lightweight seats and trim pieces. Most of the interior pieces in cars have unessessary filler material that adds a huge amount of dead weight. It's not uncommon for tuners to remove 300 lbs or more from their interior by switching to lighter seats and other stuff.
One particularly egregious offender is the interior of the Ford Crown Victoria. A while back, two cops got into a car accident, and the dashboard fell and crushed both their legs.
I'm taking a more personal approach by removing 8lbs of spare tire off of ME in the last two weeks. I'm gunning to drop another 30 by mid-summer.
Depending on the car, a person could often save as much as 20 lbs per seat (or more) by going to racing buckets. The cheapest ones from RCI weigh less than 8 lbs. each, but they're not really race legal in many competitions (if one cares) as they are made out of molded plastic. A more robust and comfortable tube frame seat can be had for about $200 each and usually weigh in the mid-teens (vs. upper twenties to upper thirties for most stock seats).
And as far as tar removal is concerned, it's best to take advantage of cold weather. Often some light impacts with a mallet are enough to shatter chunks of it off with relatively little effort. Otherwise you'll need to scrape, which is a messy affair at good scraping temperature.