When installing a new or replacement lock, be sure the keyway is positioned so that the cuts of the key will be inserted upward. This will reduce the chance of dirt, dust, and water (which can turn to ice in cold weather) settling in the lock mechanism creating a lockout or stiff operation of the cylinder.
For a high level of security, select padlocks with pin-tumbler locking mechanisms, shackles of high-tech alloy steel to provide pull strength, and special weather-resistant plastic covers to stand up to the elements.
Just about everyone knows to apply light penetrating oil or silicone spray to the hinges. But to really rid your door of squeaks, use your hammer and pry bar to lift the hinge pins up an inch or so. Apply the oil or spray to the top of the hinge, then wait a minute or two before you tap the pin back in place.