ITHACA, NYMichael Pelske changes his bed sheets after every breakup, the 24-year-old bicycle messenger announced Monday. 'I'd never bring some woman I just met home to a set of filthy sheets,' said Pelske, who changed his sheets Saturday before hitting the bars following his break-up with Linda Keely, his girlfriend of four months. 'But then, a few weeks into the relationship, you start to let things like that slide.' Pelske's cotton-twill, 180-thread-count, light-blue sheets have been washed 13 times since his mother bought them for him in May 2001. 