Hi, you dont say if you've fallen off it or kerbed it at all and have sustained wheel/tyre/suspension damage? If the problem has grown worse over time then its possible the suspension and/or swingarm bushes have worn or are loose, put bike on centre stand if it has one, stand/kneel to one side of the bike, grip the back wheel and try to move it from side to side by pushing & pulling, if anything is worn you will see some movement. If the bike is wandering, then try loosening off the steering head bearing adjustment by no more than 1/8th turn & see if there's any change, if not you may need to dismantle to check the bearing races.good luck.
Chris P.Duck, December 2010