Shave the insides. If you have an aluminum bat, the inside is hollow; this means that you can take off the end cap and shave off some of the inside walls, making the bat lighter.
Compress the bat. If your bat is made out of composite material, you can compress it or "break it in." The easiest way to do this is to use the bat for somewhere between 150 and 300 pitches. This will cause the composite material to compress a little bit, allowing it to hit the ball harder in the future.
Make an illegal bat appear legal. Most softball leagues have rules about which bats are allowed and which ones aren't. On eBay you can purchase the decals for legal bats and put them on an illegal bat, allowing you to use a bat which is specially designed to hit the ball harder.
Weight the bat. If you take off the end cap of a softball bat, you can glue a small weight to the end of the bat's inside. This will allow you to generate more torque with the bat when you swing it, causing you to hit the ball harder.
Shave the handle/bottom of the barrel. Shaving off portions of the outside of the bat can be difficult, but if done correctly you can lower the weight of the bat while still maintaining the integrity of the "sweet spot," which is found in the middle of the barrel. You will want to buff and polish the reduced sections so that they do not appear to be altered.