If you're going to purchase something, be honest with yourself. What do you hope to gain out of the purchase?
If you're looking for a new red zip-up sweater that will keep you warm, that should be your criteria when looking for a sweater in different stores. You should then find the sweater that matches this for the lowest price. You can find this on the clearance rack of a discount deparment store. It shouldn't matter what brand it is - as long as it's warm.
However, if you're looking for a sweater that will make you feel modern, chique, and trendy (a brand name or designer label), let that be the criteria. Find the biggest, best known name, again for the lowest price. You can find Gucci, Prada, Louis Vuitton, etc at outlet stores, eBay, Craiglists - these are all good places to find deals.
Mature thriftiness is knowing that something can't have everything. In the above example, a warm, trendy sweater is probably going to be of less value than if you were looking for one with the individual qualities. Set a priority and stick with it before you checkout.