To unlock Extreme difficulty mode beat the game once.

Dog Tag Know How
Dog tags are hidden on many of the soldiers. If an enemy has one, youll see a glowing spot on their chest where the tag would hang.

You'll need to surprise them at gunpoint (Hold Square when near them), then walk over (hold L1 while holding square) to stand in front of them. Point the gun at their face, and eventually they'll shake with fright and drop the tags.

Once you pick them up, you can view them from the title screen in the Special Menu. To obtain the tags from the 'persistent' soldiers, do the above, and then Shoot them in the hand or near their face. That will get them to co-operate.

Obtain Bandana (Unlimited Ammo)
Clear the game on Extreme Mode (If you just clear the Tanker Mission, it will become available only on the Tanker Mission. Same goes for the Plant Mission).

Obtain Digital Camera
Clear the game on Normal Mode (If you just clear the Tanker Mission, it will become available only on the Tanker Mission. Same goes for the Plant Mission).

Unlock Alternate Title Screen
Clear the game on any difficulty setting.

Unlock Separate Missions
Clear the game on any difficulty level and you will be able to play the Tanker and Plant Missions separately.

View Dog Tag Locations
Clear the game on any difficulty setting and you will be able to view the locations of all the dog tags. Go to the Special menu, then Dog Tag Viewer. When you highlight a missing dog tag you will see it's location at the top of the column.