You can use turbo (R3) and jump (L1) to like power jump/climb a hill or cliff. This only works on some of the hills. This is easy to do at most of the cliffs with inclines.
Elite Difficulty
Beat the game and save the profile.
Extra Armor
When confronting guards in almost any stage of the game, headshots are always best. They kill in one shot and leave the body armor undamaged. If you need to use multiple bullets, the body armor collected after searching the body will be damaged and less useful.
Health Pack
The health pack (default is Left) never runs out. This is basically the same as having infinite health. If you are ever injured, no matter how badly, use the health pack. The only catch is that you need to be alone, as it takes some time to actually heal. After a firefight, find a secluded room or corner and press Left.
Throw Grenades Through The Floor
Choose Rooftops in multi-player mode. Many other maps will also work, but this is the easiest. Look at the floor directly down, then throw a grenade. It should disappear. The grenade goes through the floor and blows up on the floor below it.