After checking it on the outside, I field strip the gun every so often, and look at the innards. Sometimes rust decides to go after parts inside, rather than outside. Alloy and polymer frame guns can be notorious for this. Aluminum and plastic don't rust, but the carbon steel parts inside such as springs, sears, etc. certainly will, and are easy to forget.
All guns, whether stainless, coated, or blued will need this. Just taking the time to take it out, look it over, and wipe it down will do wonders to keep it in order. I spray into the action with Rem-Oil until it drips out the other side on all my guns that I know are going to be away for a while without being used. I have found that Rem-Oil leaves a coating that is safe for both blued steel and wood, and polymers of course, and that it doesn't attract dust or dirt.
For a gun that's going into storage, there are moisture protection bags to put them in long term. Dehumidifiers for your safe are also available, called the Goldenrod that you plug in, and there are several types of silicone moisture absorption systems that can be refreshed periodically by baking them in the oven if you don't want to plug in your safe.
As an example of how important it is to check guns inside, a federal marine unit recently found out their HK's were great in every respect, except their hammer spring, which rusted easily under saltwater conditions. The gun looked fine outside; that was until they pulled the trigger and heard a "crack" as the spring broke. Now it was a nice German paperweightâEUR¦Once they realized the problem, they created a maintenance schedule commensurate with the needs of the pistol
The goal is to keep your friend up and running for a lifetime. It's not terribly hard if you give it some periodic TLC.