Easy way to beat walkers:
An easy way to beat walkers is to have a machinegun and flamethrower

A big Boom!!!!:
On the second level (when you get out of the helicopter for the second time). You will find a man in the building. heal him and tell him to follow you.after you get him away from the place he was you will see two canisters shoot them two times. after they explode use your fire extinguisher to put out the fire. In this room you will find ammo and a flame canister with one reload.

Simple Strategy 1:
Always keep your team members well stocked with arms and ammo, even if they are infected. It is easier to kill an infected member and take all his stuff then it is to backtrack trying to find more ammo.
Don't waste your medkits that determine DNA. If a team member is infected it will manifest itself in due time. Infact it seems to be scripted. I did a test on a member and it showed nothing. Then 5 minutes later he was a nightmare.

1st boss strategy:
Under the warehouse through a door is your first real boss. As soon as the cut scene has ended keep yourself to the left of the room keeping your back against the door. There is a tentacle that pops out of the wall on the right. Shot gun the hell out of the boss then use the flame-thrower on it until it collapses on the floor.
You will have to shoot the little spider things as your killing the boss because there is a hatching bubble just the other side of the boxes your standing next to on the left of the room.

Hidden Trailer:
Wait about one minute at the main menu to view a longer, uncut television trailer for the game.

Persuade A Person:
When someone's trust is in the orange and they will not do anything for you, enter first person mode and aim the gun at their head for three seconds. They will now do things for you without good trust.

Get A Doctor To Follow You Into A Fight With The Thing:
Use the following trick to get a doctor in the same room with you during your battle with the second giant Thing. Walk to the large garage door. Get the doctor close to the door as well. Then, tell him to not follow you anymore. Push him as close to the door as possible; and close enough to you that when you flip the switch to open the door he will not move. Before you flip the switch, tell him to follow you again. Flip the switch and when you are shown in the locked room with the second giant Thing, he should be beside you. Use him to heal you during the fight with the Thing.

Get Whitley To Follow You:
The following trick requires Stun Grenades (found in the back of the room with Boss 3), Blood Test Kit (found in the same room), and the Grenade Launcher (in the save room down the hall from Boss 3). Starting after third Boss fight (hanging tentacle monster), backtrack your way out of the weapons lab, back up the air vent, and to the "mine" elevator. As soon as you reach the surface, strafe left, and equip both the Grenade Launcher and Stun Grenades. If you step forward at all, it triggers an intermission sequence you have to die and start the level over. Enter first person mode and aim up the corridor, parallel with the bottom of the last metal wall panel. Launch a grenade and quickly walk forward. The intermission sequence will trigger, but Whitley will stand still instead of running off. Now, quickly run ahead of him (preferably block his path to slow him down) and use the Blood Test Kit. He now trusts you enough to follow orders. Order him to follow you (otherwise he walks around the corner triggering guards to attack) and give him a weapon. You have now backup. He only lasts a brief amount of time, and will disappear roughly halfway up the snow corridor, which appears scripted. However, every bit of backup firepower helps in that spot.

Skip First Boss Battle:
When you encounter the first Thing Boss, hold Up/Left and you should run under it before it can hit you. This will also aim you directly at the junction box. Immediately start fixing it. As soon as it is almost fixed, hold Right to run through the door. Since he will still keep fixing it as long as you release the button, with luck it will be fixed before the Boss attacks for the third time. At the intermission sequence you tend to take off in a random direction, so this may take a few attempts -- save immediately before trying this. If successfully completed, you will only lose only about half a bar of health and no ammunition, making the next area much easier.

Defeating The Second Big Thing:
Go into to his garage. Fix the electrical panel that operates the hoist. Step back towards the middle of the big door. There is an opening between two of the crates. Using your grenade launcher in first person targeting, send in a grenade. If the Thing turns its head toward you, then you have him targeted. Continue sending in grenades until he begins to burn. You can send in a few more if desired for extra fun. Run into the Thing's room and on the far wall from the big door is a switch. Throw the switch and watch as the Thing dies. You will probably take one hit from him.

Shutting Doors:
Always shut the door of the room you walk into, if you have the chance. For example, on the level where you have to kill all the walkers, a walker will enter from the front door if you do not shut it.