Cold sores are extremely contagious when they exhibit their trademark weepy appearance. At this point, they're shedding viruses that can cling to all sorts of objects -- those that touch your mucus membranes, like toothbrushes, can become repositories that cause reinfection later. Get a new toothbrush after the blister has formed and again after the attack has cleared up.
But toothbrushes are by no means the only product to beware of once cold sores risen their ugly little heads.
Don't reuse shaving tools, like razors, if you suspect they've come into contact with the virus. Any towels that touches your face should be laundered after every use; otherwise you might be wiping the viruses all of your face each time you dry yourself.
Forks and spoons that conceivably touch your lips should never be shared, nor should cups or bottles or anything you sip from. And of course, no lip products of any kinds should be shared among people suspected.