While playing the game, hold L2 + R2 and quickly press Right(2), Down, Up, Left(2) to unlock all weapons.
When flying a helicopter in Multiplayer mode, if you start to get laser targeted (not GTAM), fly as fast as possible, and at the same time get as low to the ground as you possible. If done correctly, the missile will hit the ground and miss you most of the time.
Unlock the Frag Grenade weapon challenge, then successfully complete all rounds and kill all enemies through round ten. If every enemy is killed, you will go into round eleven. In this round, the soldiers move and create the shape of first an "E", then an "A" as a reference to EA Games.
When doing the Hotswap Challenge, it is easier if you move towards the marker to make more "Hotswaps" available. By doing this, you will not lose 1,000 points.
When doing the RPG Rocket Challenge, the further you are away when you blow a vehicle up, the more points you get. An easy way to get points is to take a vehicle's shield down at close range, hotswap to a further away RPG man, then finish the vehicle off.
At the beginning of the Air Traffic Control level, you will start out as a sniper. Directly behind you will be a chopper. Turn around as soon as the level begins to get a good shot. Shoot the pilot and the chopper will fall a short distance directly in front of you. Make sure you hit the pilot, and not the chopper or it will explode. Note: It may require a few attempts to hit the pilot. You can then get in and shoot out the turrets with the chopper. When the jeeps come by, hotswap to one and go directly past the turrets and into the helicopter inside the heliport. You will not have acquired two helicopters, making this level very easy.
In "The Nest" map in Conquest mode, go to flag 4. Drop down the side of the building and stay close to the wall so you do not slide down in between the mountains. You will see two windows on the building if you walk towards flag 5. Along the side of this building, go to the window on the right. Use a shotgun and run towards the window. Shoot at it and you will find yourself under the flag 4 building. Additionally in "The Nest" map in Conquest mode, stay up at flag 4. Take the stairs up and go through the hall to the right. You will be at the cliff again, but go left and keep going until you find a room with only a chair and table in it. Then, turn so you run behind the chair. Run in between the wall and the chair and lay down to be under another building.