Duplicating items and weapons
Start a co-op game with at least one other player. Drop the item(s) and/or weapon(s) you want to duplicate. Then, leave the game through a non-saving method, such as signing out of your profile, going to the dashboard, or resetting the system. Since the game did not save after you dropped the item(s) and/or weapon(s), you will still have them, and the other player(s) who picked them up will also have them.

Easy loot and money
Every time you load a saved game, the chests and lootable objects restock. Save the game near a place where there are multiple weapon chests, such as in the town of New Haven. Collect all the loot in them, then save the game. Reload the saved game, and collect all the loot again. Keep the loot you want, and sell the loot you do not want. Repeat this as many times as desired. Note: You get more money and better loot when there are more than one player. This is best done with four players.

Easy rare weapons
Save the game near a location with multiple weapon chests, such as in the town of New Haven. Then, host a game, and have three other players join. Collect all the weapons in the chests, then exit out of the game, and host a new game at the same spot. Repeat this process as many times as desired. You should get quite a few rare weapons using this method.

Easy experience and weapon proficiency
Use the following trick to get easy experience and weapon proficiencies, as well as good weapons and money by selling loot from all the dead enemies. Once you reach "The Middle Of Nowhere" area, you will eventually get the "Circle Of Slaughter" quest. When do you do the quest, enter the arena, and kill all but one enemy (preferably a little one). Do not kill all enemies or the quest will end. Also, do not die or you will potentially lose a lot of money. After killing all but one enemy and collecting all items desired, exit out, and the game will save automatically. Reload your saved game, sell all desired items through one of the nearby machines, and repeat the quest as many times as desired. Note: There are three rounds; the last one is called" Final Round". Do not finish it if you want to repeat this trick.

Easy experience
Successfully complete the game, and try to be around level 40 with your character. Then, start a split screen game with your high level character and a new character that you want to level up quickly. Find a location with a lot of enemies that are easy to kill (for example, the Lost Cave with the crabs and leveled up Skags). Make sure your high level character was saved in the area you wish to go, and load him or her as the host with the other character as the second player. Leave the low level character by the New-U Station, and use your high level character to kill all the enemies. Your new character will gain levels almost instantly. This will begin to slow down around level 28, but you will still be able to get them to around level 35 relatively quickly. You will now be able to begin a new game with your new character at a very high level and play through the game easily while collecting more and better loot. This is a great way to play with the other three characters and start at a high level.

Extra item space
Create a second character for use in Co-op mode. When you want to store an item, start a Co-op game with your main character and secondary character. Transfer the items you want to store to the secondary character until you need them, then transfer them back.

Extra SDU Backpacks
Sometimes during your second playthrough and beyond, you will get an extra SDU Backpack after repairing a Claptrap even after you have reached the 42 maximum allowed slots in your inventory, which is not supposed to happen, and is a known glitch in the game. Thus, before you turn in a repair kit to rescue a Claptrap, exit the game to save. Then, turn the repair kit in to the Claptrap. If you do not get an SDU Backpack, leave the game through a non-saving method, such as signing out of your profile, going to the dashboard, or resetting the system. Then, try again until you get an SDU Backpack.

Claptrap locations
The ten Claptraps can be found at the following locations. When you repair a Claptrap, it will reward you with an SDU Backpack, which adds three extra slots to your inventory.

Crazy Earl's Scrapyard

When you enter the area, go left at the "T" intersection. Fight your way through bandits, and you will find him in one of the camps. To get the repair kit, just jump on the washing machine to get onto the platform. Kill the bandits, and grab the kit.

Crimson Fastness

You should just run into this Claptrap. He will be next to some other dead Claptraps.

Krom's Canyon

When you enter the area, go right. The Claptrap is (from the perspective of only using the right side of the map) at the north west part of that canyon.

New Haven

The Claptrap is in the town right next to the bounty board.

Old Haven

The Claptrap does not spawn until you have completed the quest to save the Claptrap from the cage. Once you have completed that quest, enter the area, and take the first right. Follow that street, as if you were heading towards the base of troopers that have the two turrets. He will be in a house on your left, before you turn the corner to head towards that base. His kit is on the rooftops.

Sledges Safehouse

The Claptrap is in the north west part of the map, near some rubble, and close to a stairwell; listen for the Claptrap's moaning. His repair kit is above you, when you are standing on the waypoint.

Tetanus Warrens

The Claptrap is north of where you start, before you get to the fork in the road, on the left side. The repair kit is further north, staying left, above some metal structures in the first corner, farthest north west.

The Lost Cave

The Claptrap is near the point where there is a "T" intersection and a bandit outpost area. When you are looking at the outpost as you enter, the left path leads to a dead end, and the right path leads to the spiral area. The repair kit is up the hill (following the rail tracks up and around) and just behind the second or third building in some pipes.

The Salt Flats

When you enter the area, go towards the huge piece of saw machinery. Once you enter that area (from the right side), take your first right, and you should see the Claptrap.

Trash Coast

When you enter the area, turn right, and look at your map. You will see a fork in the road, go left, then make your first right. The Claptrap is in the middle of a bandit encampment right next to where a bunch of lava crab worms are. If you find the shack on the ocean, it is straight west of that and a little south. It is almost right in the middle of the southern potion of the map. The repair kit is on top of the pipe where the diamond leads. Follow it back to the trash pile, and climb onto the pipe to get it.