Leeches can be kept for many weeks in cold water. That is why you buy them in refrigerated containers. Water temperatures between 35 to 40 degrees are ideal. After buying your leeches, most people put them on ice until they arrive on the lake. Some fishermen keep them in the cooler even while fishing--digging into the cooler for a leech every time they need one.
When I arrive at the lake, I take my leeches out of the cooler because I want them to warm up so they will be active. I add lake water periodically to keep the water close to the temperature of the lake. A small styrofoam cooler works well.
Black leeches that are swimming are the ones I use. The brown leeches, especially if they are curled up on the bottom of the container, are the ones I give my brother-in-law, or mother-in-law. I am not kidding and I am not alone in believing that the dark leeches work better.
When you watch a leech swim, the small end or head, is forward, so why do most people hook them through the suction cup, i.e., foot. This method works ok if you're using a bobber, but when you are trolling or drifting, it's unnatural to drag it backwards and the leech tends to curl up. I hook mine through the head and they seem to swim better. It is definitely a more natural presentation.
Use small hooks, number 8 or 10--light line--6 lb or 4 lb and your leech will act more natural and trigger more bites.