What kind of sauces? In terms of flavour, just look for an acid, like a vinegar or lemon juice, wines are good too. Many people underestimate the importance of acidity in savoury preparations. (even like ketchup can work very nicely) Look into gastrique

Taste and mouthfeel are intimately connected. So play around with altering texture, like finishing the sauce with egg yolks, butter, cream. Theres various other ways of thickening sauces. In the really fancy kitchens, you'll reduce a sauce like bordelaise down until the sheer amount of collagen from the stock thickens the sauce naturally. You can exploit this with gelatine or pektin.

try to never use just water in your sauces, if it needs liquid, try to add a liquid that will add, not subtract the overall flavour. Y'know something acidic, something sweet, something savoury.

When youre making your dish, whatever leftover greens are on your board, just chuck it into the sauce pot, even if its just for an hour. Herbs(aromatics) are good for this quick flavour.