If you're buying them at the grocery store, consider the time they've already been off the plant. They are covered in wrap which causes condensation, shipped in a truck, etc. Try to buy as fresh and local as possible, only buy the amount you will eat in 2 or 3 days. They need to be kept dry and have air.
Some people freeze them or you can slice them and sprinkle some lemon juice and sugar on them (and keep in fridge until ready to eat), and it helps inhibit the mold growth.