I always have a needle and thread and a roll of duct tape. Fabric glue would be good to have packed too.
I second that. I usually carry about 30 yards of heavy duty nylon thread and 3-5 med-large needles for self repair. I also carry a small tube of 'Fabritac' (fabric glue) as well as a small roll(30-50 yds) of industrial strength duct tape.
I've also found that if you have the load bearing seems reinforced before use it goes a long way to increasing the life expectency of any bag.