3. Autosubscribe to staples
Many sites offer discounts for autosubscribing to things like diapers and dog food. If you know you’re going to buy it again from the same place next month, you might as well save a little in the process. 

And, as a side note, if you really want to save money on diapers and don't mind doing the dirty work, cloth costs a fraction of the price of disposable over the course of a couple years.