Phishin has good information. When salting the water, be sure to use aquarium salt and NOT iodized salt. Iodized table salt will kill your fish.
Salt is good because it helps baitfish maintain electrolyte balance and it cuts down on fungus and other infection.
If you are just keeping bait for a couple days, the biggest issue you will cope with is ammonia buildup. About the only way to deal with it is FREQUENT water changes. This can be a real chore but it's necessary. You also need to maintain good aeration and keep the water chilled (freeze milk jugs full of water for this). A filter is also a good idea.
If you plan on keeping bait for an extended period (a month or more), you can set up a filter and establish biological filtration that will take the ammonia out. This doesn't happen overnight though - it's like keeping a giant aquarium.
Before I got into fly fishing, I kept a 300 gallon tank for shad, bluegill, shiners, and stoneroller minnows in my garage.
I've still got that 300 gallon tank and all the filtration stuff in my garage, but I haven't used it in a couple years. One of these days I'll probably try to sell it but there's obviously a limited market for something like that.
bd