In the case of the 870......nothing.
The Remington 870 is the closest thing made to a "bullet proof" pump gun.
This is the reason why 95% of all American Law Enforcement use the 870.
There are things people do to the gun to enhance "reliability or durability", but in truth, most of this is nothing more than an urge to customize a gun.
Some people rush to replace the plastic magazine follower, "so it won't break, and jam the gun", but research has failed to turn up any verifiable cases of the follower breaking in the gun.
Since the follower isn't under any strain while in the gun, it doesn't break.
The "cheap plastic" magazine follower has been used in Remington Police guns for MANY years, with absolutely no problems.
If several hundred thousand street cops over 30 years can't break something, it's not an issue.
The best thing that can be done to an 870 is to shoot the gun. This will break it in, and the action will smooth up as it's used.
The simple fact is, unless you have a rough or defective 870, there isn't anything that can be done to it that will REALLY increase reliability or durability.
The rest, sights, stocks, extensions, slings, etc are personal preference or attempts to build Hollywood Range Toys.
There is no more reliable or durable a gun right out of the box than a Remington 870 Police.