Oil_pan is right, the way to save gas is not by spending money on lighter parts. With the exception of parts that already need replacement, in which case go for it. You won't see a very large benefit from weight reduction unless you do a massive amount of low speed driving. Most of your weight reduction comes from deleting things altogether such as a/c, power steering, cruise control, back seats, passenger seat, removing sound dampening, interior paneling, spare tire, etc. There really isn't a whole lot of weight to be saved if you are trying to maintain creature comforts and keep the interior. Sure you can throw on some lightweight rims and tires, a lighter battery, carbon fiber hood, aluminum control arms, racing seats, a lightweight flywheel, the list goes on... All of these things will reduce weight, but it's cost prohibitive because they will cost far more than the gas you will save by making the modifications.
Here is a link from a Civic forum that talks about weight reduction.
Weight Reduction 101
Forgot to mention.. You could drive with only have a tank of gas, but that only saves you 37lbs.
Edit: WOW! 2 posts before I could hit submit