There are certain times of year when items go on sale due to holidays, events and other celebrations. It is during these times that you should stock up on those products. They are likely the cheapest they'll be all year, so it is best to buy multiple items in order to sustain several months. Shop like a seasonal pro by taking advantage of the deals you'll find at each time of year.
During the late summer months, you will see several items go on sale and coupons to go along with them to make them super cheap.