As your meal cooks, the stock simmers. At the end of the meal, strain it and return to the fridge. Stock should always be simmered and never boiled. The goal is to coax the flavor from the trimmings. Boiling results in an overall deadening of flavor and will also destroy many of the vitamins that the stock would otherwise be rich in.
Onions are often one of the best ingredients in stock.
Cauliflower is good, as well, especially in creamy soups where it is pureed with potatoes. Generally, to keep the stock flexible enough to include in a number of different ways, I refrain from adding spices. But if you had a plan in mind, leaving the stock to sit with the herbs overnight, enables its flavors to penetrate all the ingredients. Before adding spices, though, try throwing in a dash of salt, which may be all you need. Soy sauce is also a good addition in some stocks.