To get Emblems from defeated ACs, press START+SELECT when the victory slogan appears.

To earn bonus parts, get an A or S on any mission.

To boost faster press both L3 and R3. You should boost forward at twice the normal speed until you run into something. However, you can not stop until you run into a wall or other object, so there is risk of using all your boost.

When you encounter the mission to protect the new shipment of weapons while the plane is re-fueling, go near the plane when it lands near your Core. Go behind the plane when it stops and take the gun. You will fail the mission but will have the weapon.

Another hidden weapon is in the mission "Eliminate Rioters". At the end of this mission you will run into an AC named Struggle. While in that room destroy the boxes until you find the gun.

The hardest weapon to get is in the mission where you must destroy the prototype base. In this mission you must sneak around without being seen. While you are walking around Metal Gear style, find the crashed airplane wreck, then go west from there to find a cave. The first thing you see inside is the weapon.