Since it is prohibited to possess chametz on Pesach, any chametz left undisposed must be sold to a non-Jew. [Any chametz remaining in the possession of a Jew during Pesach may not be used, eaten, bought or sold even after Pesach.]
Therefore, all chametz that will not be eaten or burned before Pesach and all chametz utensils that will not be thoroughly cleaned by then should be stored away.
The storage area should be locked or taped shut to be leased to the non-Jew at the time of the chametz sale.
Since there are many legal intricacies in this sale, only a competent rabbi should be entrusted with its execution. The rabbi acts as our agent both to sell the chametz to the non-Jew on the morning before Pesach starts and also to buy it back the evening after Pesach ends.