You can try removing the toner cartridge and shaking it from side to side a few times. This distributes the ink more evenly. Failing that depending on how long you have had the printer, the only other option is to replace the imaging unit or "drum" as I would describe it.
OR - there is a NEW printer here for £139.99 - I don't know the difference if any there is with your current printer but worth taking a look at. link text