When you are playing as John Morgan, enter rage mode. John will pull out a staff, which is separate from his standard inventory. If your inventory is full at that point and another weapon pickup is nearby, you can quickly select that other weapon to replace the baton that John is carrying. That will add an additional weapon to your inventory, so that you are now carrying more than the usual maximum of eighteen weapons. You can continue to expand your inventory in this manner, by activating rage mode and swapping out the baton for addition items as your inventory swells.
Cars
If you jump onto the hood of a car and then crouch and aim right you will be able to kill zombies without them being able to touch you.
Chrome 2 reference
Play the "Born To Be Wild" quest. Enter the hotel and talk to Mike Davies. Then, move to the next room with the workbench. Enter the next room with a computer to reboot. Look at the wall to the side of the desk with the fan on it, to see a Chrome 2 poster.
Easy "How many days exactly?" Achievement
Disconnect your online connection, go to the system settings and advance the system date by one month. Resume the game while offline to unlock the achievement.
Easy "Military-Industrial Complex" challenge
While doing the "Bitter Return" mission there is a mounted machine gun on the balcony of city hall. Mount the machine gun and fire away. Every bullet fired counts towards the "Military-Industrial Complex" challenge. Keep shooting and you will complete all four levels.
Easy "Oh, No You Don't" Achievement
This can only be earned with Sam B, and after unlocking his Tackle ability. Play Chapter 16, "Locked Up". Once you get to the checkpoint where you have to "Find another way to the cell block control room", you must go around to get to the control room. When you open the door to the cell blocks, there is a stairway going down and there you will see the Ram. Let him follow you upstairs and go back in the hallway. Leave the door open. The Ram will try to ram you down, but he will not be able to get past the door. This is where you can weaken him until his life is close to dead. Then, deliver the final blows with you Tackle ability.
Easy "Rootin Tootin Lootin" Achievement
Search for orange colored weapons. Purchasing them or receiving them as quest rewards will not count. They must be found in crates or looted from enemies you kill. Consider using Purna as acharacter, as she has a skill with three levels in her survival tree that allow a better chance to find better loot in increments of 5%, 10%, 15% respectively. Drive around the Resort in Act I and search as many named locations as possible (bungalows, gas stations, and even the big resort). Note: Chest rarity does not seem to affect on item rarity.
Easy champagne
In the Slums there is an abandoned building at the bottom of the map. Once cleared out, you will be able to quick travel to there. In the back of the building are two rooms. Each has a fridge that may contain a bottle of champagne. If not, quick travel to another area then quick travel back as many times as desired.
Easy experience
Reach Ope's Cave in Act 3, then use the "Duplicate Weapons" glitch to copy your most valuable weapon. Sell the weapons to a vendor to get a lot of money, then buy Oleander from a vendor if it is available. Taking as much Oleander as you can buy, fast travel back to Ope's Cave and turn it in to the tribesman that give you the Continuous Event "Dreamtime". Each time you turn in five Oleander Blossoms, you will receive $1 and 1,000 experience points. Repeat this as many times as desired.
Easy money
Use the following trick if you want to modify, repair, or create a weapon but are short on money. If the craft table is inside of a store, search the cash register. Occasionally you can find a couple hundred dollars.
When you reach the city of Moresby and have access to the warehouse, there are four weapons located inside, the wrench, baseball bat, hammer, and knife. Selling those four items at the church will generate approximately between $400 to $2,000 per trip. Repeat the process as many times as desired.
Easy 'Swing Them Sticks' Achievement
An easy way to get this Achievement is to run over zombies in a car. As long as you have the analog control scheme selected any kills you get will count towards the 150 needed.
Getting kills without dulling your weapon
Killing zombies takes a toll on your weapon. However when it is on its last legs it is not completely useless. Throwing your weapons can damage enemies without dulling the weapon. If you have a weapon worth keeping, but it is almost used up, keep throwing it at zombies until you can repair it again.
Hidden provocative photographs
In Act 2, enter the mayor's office in City Hall before it gets attacked by zombies. Knock down the painting on the very back wall in the room behind the desk. You will find three photos of a semi-nude blonde woman.
Killing humans
This trick works best if you play as Logan. Humans can use weapons, and so can you. However when it comes down to a gunfight you are both evenly matched. A good way to kill humans with little effort is to throw modded weapons. However, there are a few exceptions. The weapon Must be a sharp weapon. Blunt weapons only damage them slightly. The military machete works best. The weapon must have either a Poison mod or a Shock mod. Fire will not do anything. Make sure you upgrade the weapon to at least level 3. You may want to mod a powerful weapon because it will have minimal affect against stronger humans. Keep your distance when throwing, and have plenty of ammunition. This is one reason why Logan is better for this, because thrown objects can come back to him. However, if you have a clear view of the target, throw the weapon and they will die instantly.
Killing large groups of zombies
When you encounter large groups of zombies or have ran and have a lot of them following, use the following trick to deal with them. Look for a nearby vehicle and sprint to it. Jump on top of it and crouch on the hood. While there you can still hit the zombies. Note: Do not do this on the roof of the vehicle; you will not be able to hit them, and they can still hit you rarely. Note: Use the analog style of fighting to do more decapitations and swing your weapon faster.
League Of Legends reference
Shortly after the Lifeguard Tower (Exodus) is completed, there is a quest giver in a bungalow to the left of the main road named Annie. She initiates the quest by saying "Have you seen my bear?" and asks you to retrieve the teddy bear for her. In League Of Legends, there is a champion also named Annie. In that game, Annie's catchphrase is "Have you seen my bear Tibbers?"
Lost reference
Do the quest to clear out the Lifeguard Tower at the start of the game. Clear the garage of the few zombies there. Go inside and you will hear a radio broadcast of an Oceanic Flight in an emergency descent to an island due to engine failure.
Never Dull Your Weapons
You will never dull your weapons in any level if you throw them.
New Game+
When you have completed the campaign any money and inventory (blueprints, items, weapons) you have acquired is carried over to the next game (or 'next play'). This extends to the items you have left with the 'item bank', the NPC Jin.
Pimp My Ride reference
Do the quest near the end of the Resort level, where you take the big truck to the mechanic to turn it into a homemade tank or fortress. While talking to him, just before you have to defend the shop while he does his work, he will say that he is going to "Pimp your ride".