Always use the vegetables and seasonings sparingly — better too little than too much.
If leeks are included in the mirepoix, great care must be taken not to brown them when softening the mirepoix. Better to throw it out and make the stock without any vegetables at all. The bitter taste of a browned leek can never be expunged from further preparations.
For white stocks always wash the bones thoroughly under cold water first to remove any blood.
When making brown stocks always use a receptacle large enough so that you can brown each piece of meat with care and attention.
Add enough cold water to cover the bones easily. If insufficient water is used less flavour will be extracted from the ingredients. The aim is to dissolve the flavours into the water. Many cooks simply throw the meat and bones into a pot, let the stewing mess boil away until it is dry and browning the bottom of the pot, throw in some tomato paste and consider that sufficient. It may be if you are making a pot of goulash for a meeting of Belgian and Swiss humorists but not if you are making a fine gelée for yourself!
Never boil a stock unless it has been strained of all solids and you are reducing it.