Check into Mother Base frequently (and hit everyone!).
The actual area of Mother Base is pretty boring, but you’ll still want to check in every once in a while to see optional cutscenes involving some of the important folks on your team. You can also smack around some of your troops to build their morale—because nothing boosts troop morale than getting beaten up by the boss.
Capture as many people as you can.
When you’re out in the field, you can send just about anyone back to Mother Base to be brainwashed and recruited into your army. This is a pretty easy process: just knock out an enemy and attach a Fulton balloon, which will send them right back to base, as long as weather conditions are OK and no enemy soldier spots you and shoots down your balloon.