Seriously, the only real way to remove weight is to remove components....
You could swap the mechanical hard drive for a SSD. This would save some weight.
You could remove the Optical drive entirely. This also would save some weight.
You could switch to only 1 stick of ram. Again this would save some weight (although would affect performance).
You could also remove the battery and replace with something smaller (i.e. maybe a battery from a 13?). This will lower your battery life.
The first three might shave a half pound off, the last one maybe another half pound.
In the end either get an MBA and hook it to a 27" screen or GO TO THE GYM.
Seriously, electronic components don't "get heavier". Other maybe a little dust in side, they don't add or lose weight. Seriously....