Most effective way was already stated by Pix earlier on.
to be a bit more detailed,
a) consider using turning on Frequency Hopping, either Baseband or Synthesised Hopping, depending on the hardware your network uses.
b) Trial out BTS and MS Power Control, for actualy parameters and optimised value, refer to your vendor
c) Activate DTX, refer your vendor for exact implementation method. This will reduce the duration of transmission, and hopefully reduces interference