Flavour stocks with chopped vegetables such as onions and carrots, and a few sprigs of herbs. Make sure you don't add too much salt at the beginning if you are going to reduce and concentrate the stock later.
Meat stocks require a couple of hours simmering, while fish stocks only need about 20 minutes. Always start with cold water and bring to the boil over a gentle heat, spooning off any scum as it rises. Scoop off any fat from the surface at the end.
Stocks freeze really well, so it is worth simmering a big batch. After straining, reduce the cooking liquid until it's really concentrated. You can then store the stock in ice cube trays, saving heaps of space in the freezer.