Porcini mushrooms dry very effectively, and in their dried form are very important in the Italian larder. They are used throughout the year, not as a substitute for the fresh mushroom, but as a separate valid ingredient in recipes as a seasoning. Dried porcini have a more intense flavour than fresh, and a little goes a long way.
The quality of dried porcini depends on the state of the fresh porcini when picked. Large firm mushrooms with their caps still partially enclosing the stem are best. Sliced finely through the cap and stem, the pieces are laid out, often on newspaper, to dry in the sun. Dehydration concentrates the earthy fragrance.
Dried porcini are usually sold in transparent packets: look for pale cream-coloured stems and light brown caps. Thickly sliced, darker coloured porcini are oven-dried, which seems to affect the flavour. These are usually much cheaper, but not really worth it. If you cannot distinguish the shape of the slice, and there are lots of small pieces and crumbs, they may not be porcini.
Ideally, buy your dried porcini when you are in Italy. Many shops sell them loose, they are cheaper, and you will have a selection.