Here is everything that comes to mind. Some are more practical than others:
remove carpet
remove headliner
remove rear seats
remove passenger seat
replace your seat with a lightweight racing seat
remove engine cover
remove insulation under your hood
replace hood with carbon fiber hood
lightweight wheels
replace windows with plexiglass/lexan
remove passenger side mirror
drilled door handles
remove badges/emblems
remove interior trim
lightweight battery (braille, odyssey, deka)
hell, take anything that isn't welded onto the chassis out of the car!
Practically, ditch your rear seats and get light wheels and a carbon fiber hood and a lightweight battery. There's not much else you can do on a street car