These are game secrets that can be done after some point in the game. Follow the instructions as follows.
More Customizations
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For additional player customizations, log-on in the Lifeinvader website from your smartphone, then start "Stalking" some people, this will add customizations depending on the character you are stalking, like hairstyles, etc.
Discounted Goods
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Finish all Gun Range challenges to receive 25% on all items. You can also receive lower discounts for partially completing these challenges.
Getting Rid of Wanted Level Quick
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Once wanted, enter a Vehicle Mod Shop, you will exit out with zero wanted level stars as long as there will be no cops around. Alternatively, you can simply switch to another vehicle to easily lose the hand of the law.
Reveal Jump Location Points
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Pull up the internet browser from your camera phone, visit "junkenergydrink.com/page5" to quickly view all the stunt jump locations in the map.
One-Way Shootout
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While currently holding a wanted level, Head to haircut store in Los Santos and cover the door using your vehicle. Place yourself by the window, this makes you able to shoot through the glass hitting the cops while they cannot shoot back in return.
Flipping the bird
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Ride a vehicle, then go barehanded by choosing unarmed via the weapon wheel. Push L1 for PS3, or LB on Xbox 360 to give the middle finger to pedestrians.
Skyfall Cheat Safe Landing
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Activate the Skyfall cheat code, then perform a vertical landing face-first, this will not hurt you at all even if are normally supposed to.
Infinite Cash
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Once you have two characters available, choose any two points in the map with a Hidden Package nearby, like the one in Paleto Bay. Place both characters on those two locations. Grab one Hidden Package using one character, after getting the cash, switch to the other character and get the item as well. Keep switching both of them as this will reset the spawning of the Hidden Packages, just make sure the two locations are pretty far from each other.
Infinite Health and Vehicle Recovery
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While your Vehicle is damaged or your character is hurt, change to another character, then immediately switch back. Your health and your vehicle will be fully replenished. This only works outside of missions though.
Infinite Upgrades
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Enter a mission, then head to an Ammu-Nation shop and buy some stuff. Let yourself die and this will fail the mission. Once your back though, you will find that all of your cash have been returned and all of your purchased stuff are still tagged as "purchased".
Flipping Out
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Jump up by pressing X on the Xbox or Square for the PS3, then press B for the Xbox or Circle for the PS3 to perform a flip.