i keep them in sealed containers in the fridge. a layer of damp paper, about 10 worms and another layer of damp paper and they usually last about 5-6days if cleaned regularly and dead/damaged ones removed.
look in the Q&A forum for a topic called "wrasse and hardbacks" and have a look at "alanandjordans" bait fridge. looks a pretty good setup and and can be done pretty cheap.
you can keep them for long enough aslong as you keep changeing the water in the cat litter trays and keep the trays about 3/4's full where i live i have little choice but try this and it works very well tides my way are only good twice a month.