Do you want to save it for later or use it now?
If you want to use it within the next couple of days, step up the starter (add more wort).
If you want to use it in the next week or so, put an airlock or foil on it and put it in the fridge, then make a starter with it the day before you're ready to use it.
If you want to save it and use it more than a week from now, put it in the refrigerator and let the yeast settle (24 hours or so), then pour off the beer and replace it with boiled and cooled water, seal, and refrigerate. (This deprives the yeast of oxygen and they'll keep longer.) Then make a starter with it when you're ready to use it.