In any chapter, collect as much gold, items, and treasures as possible. Then, pause the game and quit. Save the game when prompted. Restart the chapter to find that the gold and other items are still in your inventory. Sell any of the items at the "Shop" menu that appears before the start of the chapter.
Go through a chapter while collecting as many hand grenades as possible. Then, start Co-op mode with a friend, system link, or over the Internet. Hand your partner all of your grenades and have them exit the game. After they leave, your AI partner will still have the grenades. Make sure you request them back. While you do this, your friend that left can sell the ones he took with him. You can have up to five in each slot, and they each sell for 250 gold. You will gain 11,322 gold each time you do this (if you have all your slots full). This is also a great way to stock up on ammunition.
As soon as you defeat the Uruoburos for the second time, repeat the beginning of that chapter and collect plenty of money and ammo (especially for the machine gun). Repeat this several times and you will be able to upgrade every weapon.
After completing the game, go to Chapter 5-1 (Underground Garden) and bring extra ammunition if possible. Proceed until you reach the part with all the Lickers. Do not press the button to the elevator yet. Instead, kill all the Lickers that are there, then gather all the treasures and money they dropped. Press the button to the elevator and kill the four or five Lickers that appear. Once again, gather the treasures, then quit the game and repeat until satisfied. You should make about 10,000 gold selling the treasures the Lickers drop. There are no enemies except for the Lickers you are meant to defeat, which will make for easy money for weapon upgrades.
To stockpile gold for upgrades, replay the beginning of Chapter 4-1. Start by running forward en route to the cave entrance. Once in the cave, run forward and a cinematic will play, introducing the Bui Kichwa spiders. These can be taken out easily with a flash grenade, but are also easy enough to plow through with a pistol and/or knife. Dispatch the first few spiders you see, then run past the ladder and into the small room at the back. Run to the vase, turn around, and look up to shoot a gem off the ceiling. Also, get the gem from the chest. Backtrack and go up the ladder, then take out the spider and proceed onwards down the winding cave. Just past the first few vases, knife the standing torch for a gem. After taking out a few spiders, continue forward and knife the ground torch for another gem. Once you reach the bottom, there are three gems to be found. One is in the skull on the right. The other two are on the left-hand side; one in the standing torch, and one on the ground in a skull. This takes about four minutes and yields 8,000 gold in gems, plus an additional 2,000 gold if a Bui Kichwa drops a rotten egg. You should also pick up a variety of ammunition and grenades after doing this for awhile, so it is well worth your time. Additionally, you may continue forward through the door at the bottom of the cave. If you choose to, then do as follows: run forward and jump down, then shoot at the wall over the entrance to the next room for a gem. Keep moving until you find a tomb that you can open for 900 gold. In the next room, knife the lantern for a gem and continue forward for a cinematic. After that, drop down and mow through all the Majini in your path, stopping for another gem located in the ground torch behind the masked Majini with a crossbow. After taking care of the shielded Majinis, walk into the first room and collect two gems and 700 gold from the tomb. In the next room, grab the gem from the ground torch and the one held by a mummy. Once you fall down the trick tomb, take care of the Bui Kichwa and the Majini who appear, then retrieve the three gems from the chests. Quit after this point if you came this far. This half of the trek yields 12,000 gold in gems plus additional gold, ammo, grenades, herbs, etc. found along the way. It usually takes seven to eight minutes to do this. Note: It can be frustrating to keep fighting all the Majini; you can choose to only follow the first part before repeating.
At the beginning of the Marshlands chapter, go up to the body of the dead soldier. Ignore the slate and pick up the beetle on the wall, the ruby in the mask, and the barrel contents. Then, visit the following locations in no particular order: the island hut full of chickens, sinking boat, triangle huts, and village. At the chicken island, collect the eggs, the treasure in the house, and the barrel contents on the lookout. At the sinking boat, get the barrel and the rocket launcher. At the triangle huts, collect the other beetle, the barrel contents, and the treasure on the lookout. There is also an insignia near the ceiling to shoot. At the village, collect the emerald and all the barrels and jars. There is also a ruby in a mask on a post near a hut and the treasure. Do not get any slate pieces. Quit, go to "Item Management", then sell the treasures, eggs, and anything else that you do not need. Put the rocket launcher in storage and repeat the process until you are rich and have plenty of rocket launchers to make the boss battles easier. Use the money to level up your weapons. You can earn roughly 15,000 gold each time. If you need to stock up on machine gun bullets, do the same thing in the Chapter 2-1 storage room. A blue insignia is also there. In the Caves, you can use your rocket launcher to take out the bat Boss. You will get a green gem worth 10,000 gold, which when added to all the other cave treasure, can increase your money significantly.
At the start of Chapter 2-1, you will be in a garage with two Humvees, a few breakable crates, and a machine gun. Pick up any items you wish, including the machine gun. End the game and choose "Yes" to overwrite your current equipment. Continue the game from the main menu. You will have gained your items, as well as the machine gun. From here you can sell the machine gun back for easy money, then repeat the process over again. This is an easy way to help upgrade your weapons early, as well as max your ammo out.