Cook garlic and onion until soft, then add rest of ingredients. Simmer on low 20-30 minutes. Keep stirring often to prevent burning, then refrigerate for a few days before use. This allows the flavors to blend and mellow. (You can use it right away but in really does improve with age)
A simple addition to this recipe which makes a dramatic change is to add about half a can of beer. Believe it or not the cheapest beer you can find, one of the old Stag, Falstaff, Pabst Blue Ribbon or Busch beers. A stronger hops flavor makes a huge difference.
Another variation that makes for an interesting sauce is to use Lea & Perrins White Wine Worcestershire (now called Lea & Perrins Marinade for Chicken, I think). It changes the flavor distinctly.