Start with a light car in the first place......
Seriously though, although you can make reasonable savings by removing trim etc. , percentage wise the saving won't be significant, so you are unlikely to notice the difference on the road.
I'm buying a racing battery for the Exige which will save me 5kg. Doesn't sound much but compared to the weight of the car it's worth it. It's also a third of the size and as the battery is in the boot it will make more space. I might be able to get my laptop in then... I'm also fitting harnesses and removing the seat belt, so hoping this will save a couple of kg's