MGS3's controls shouldn't be taken for their face value. While some functions like peeking in first-person mode can have use on their own, there are many functions that you might miss that can come in very handy.
Normally you can open doors by walking directly into them. Running into them at top speed will bash them open (which does not alert guards for some reason), while approaching them carefully will open them more slowly. You can also open a door partially and peek through - stand next to the door without opening it, enter First Person View, and use the and buttons to peek through the door.
Flatten yourself against a flat railing (it must be about waist-level to Snake) and press . Snake will jump the railing and hang from the other side. Good for avoiding inquisitive patrols, but in the right context, also good for making difficult shots with the pistol. Beware the camo index, though — hanging from things like tree trunks will probably lower it to 5% or less. Veterans of Metal Gear Solid 2 will probably try doing pull-ups from the ledges (+) — unfortunately, you cannot increase your grip strength this time around. Going into first person during a pull-up (++) may give you a tactical advantage when you're unsure of the enemy's path.
Crawling through grass will shift the game into first-person Intrusion mode, which means that 80% of your screen will probably be filled with grass. If you need to make a clear shot from the grass, though, hold , , and to lift yourself up from the grass just slightly to peek forward. Guards don't notice you any more than usual (unless your camo clashes with the grass).
Hiding in lockers
An effective way to avoid enemy detection. Press to open the locker then press yourself against the back whilst inside to close the door. Pressing whilst in a locker with a poster on the inside will cause Snake to kiss it, whereas pressing it whilst nothing is inside will cause Snake to knock. It's not a perfect way to be detected, however. Enemies who know that you are in the area may still check inside.
There are many ways to hide the guards. Dragging them in the shadows () is a great way to prevent other enemy soldiers from finding the bodies. Another great place to hide them is within the lockers. Dragging them into open lockers will cause Snake to list them in and close the locker. Enemies who find the enemy soldiers will either stumble and faint or trigger an alert phase.
When you see the trademark "?" or "!" above an enemies head, shoot it to confuse the enemy for a while. Small bubbles will rise from their head when successful. This prevents an alert phase triggered by that enemy and gives you about 20 - 30 seconds to escape.
Breaking enemy radios
If you shoot the enemy's radio, it will crackle and break, preventing them from being able to call in an alert phase. Unfortunately it also prevents them calling it off.