I have gone to great lengths to make slip using Robin Hopper's recipe. People in my old studio laughed that I was taking dry ingredients and making slip in this manner. It was, however, great slip and stuck to everything. I think if you google his slip recipe you should be able to find it on-line or at the very least in one of his books.
Now however, what I do is take some of either my reclaimed clay or cut off slices of the clay body I am using, dry it throughly, put it in a bucket with some water and let it slake. You don't want to add too much water but just enough to really cover the dried slices. After a few days of slaking, I then take a hand held mixer or you could use a blender and simply mix it up to the consistency I want. I try to really make sure if I am adding Mason stains that I mix these really well so no spots or speckles come through in the initial bisque firing. This is quick and easy and serves all my purposes. I store the slip and simply use it as required. For example, I will take some out of the storage container, mix it up again well and add my unique colorants bit by bit as needed. Works great for me. I am sure it would work equally well in a slip trailer but you may need to water it down to the right consistency. Not so watery that it runs but not so thick that it clogs the end of your bulb or bottle.
Good luck. I love working with slips.
Thanks to everyone for their great responses!! I'm going to start slip trailing immediately! Do you guys suggest adding mason stains over chemicals like cobalt carbonate or copper carbonate?