Most stores offer a specific time period for you to get a price adjustment on an item that has received further reductions after your purchase. They may even adjust to competitor's prices after the sale if you bring them all the proper documentation such as your receipt and a copy of the advertisement. Ask the store what their policy is regarding price adjustments before you make your purchase and make sure to get a hard copy of the policy for your records.
For stores that fail to cooperate within a reasonable time (generally two weeks are what most retailers consider reasonable) then simply return the merchandise and repurchase it at the lower price, elsewhere.
Just remember though, if you do not shop after the purchase you will never find out that the price was further reduced. Also, the burden of proof is on you so keep your receipt!