Buying:   Liver is still fairly common in meat markets, but most available in ethnic markets serving Eastern and Southern Europeans, Mexicans and Middle Easterners.

Prep:   The way they're sold around here there isn't much to do except remove a little plumbing, slice and rinse well in salted water - unless you need to kosher them.

Cooking:   Liver is usually cut into smallish pieces and cooking time is kept very short, as liver tends to toughen with longer cooking. This is, of course, impossible with Jewish recipes since the liver is already well overcooked by the koshering process (see below).

Koshering:   If you are using liver in a Jewish recipe, this procedure is necessary to reproduce the authentic flavor of the dish. If you are Jewish and want to keep "sort of kosher", the basic procedure here should do fine, but if you're keeping a hard core kosher kitchen there are a lot more details and you should consult a paper by an authorized rabbi.. The main point is this: blood is forbidden for Jews, and liver is so saturated with blood koshering by salt is considered insufficient.

1. If you're really doing kosher, you must buy your liver from a kosher butcher to make sure the animal has been killed and dismantled in accordance with kashrut laws.
2. If koshering a whole liver (this would be the case with a lamb liver), you need to make sufficient cuts through the outer membrane for the juices to flow through.
3. The liver may be salted or no. If it is, usually just enough salt is applied for proper flavor in cooking.
4. Flame is required. Preferably use a gas or charcoal grill with the flame below the grate, but with care it can be done in the broiler. If broiling, use a grate in a pan so that all liquids drain away and cannot touch the liver. In either case, you must roast the liver all the way through, turning once to get it flamed on both sides. There must be no dark brown color in the center. It is not necessary to char the surface, but some recipes do call for that.
5. The liver may now be cut in whatever way is appropriate to the recipe being used.
6. Caution:   Once a grate, pan, tongs or other tools and utensils have been used to kosher liver, they must not be used for any other purpose nor come in contact with tools used for other purposes.
7. Caution:   If broiling, you must be careful no liquid from the liver splashes on the walls of the broiler or any other part of the stove, or you stove may thereafter be used only for koshering liver. There is probably some rabbinical procedure to restore the stove to general use, but it probably involves sand blasting and a substantial donation to the temple.
8. Fortunately we Pagans and Christians can disregard the above cautions.