I have shipped fish hundreds of times. How many fish are you shipping, and how far?
There are a few options for you as far as packing goes...the two best are Kordon's breathing bags or regular fish bags, double-bagged and bagged with pure oxygen (a little zeolite in the bottom helps, too). These will then go within a sturdy styrofoam box inside a good cardboard box....the regular fish bags can be packed snugly, the breathing bages need ait around them, so they must have some styrofoam peanuts or crumpled newspaper around them. Fish should be shipped by the most direct and expeditious manner possible, so shipper, service, and times should be planned well in advance...and always ship early in the day (or very late in the day) on a Monday or Tuesday, so that they get to their destination very quickly and do not end up sitting in a depot somewhere.
If you have more specific questions, please feel free to ask...I would be happy to walk you through the process.