When you're low on health and not in the middle of a mission, switch to a different character and then immediately switch back to have your health fully restored.
To make easy money in the stock market, invest in a company and then target the competing company. When you do something in Grand Theft Auto 5 that affects a company (like robbing a bank or gas station, or destroying a plane), its stock price will decrease by a certain amount depending on the quantity of damage you caused to the company (the more problems you cause the company, the lower the stock price will go).
While playing as Franklin, you can do assassination missions for Lester. Before completing one of the assassination missions below, switch to each character, and invest all your money in the listed company. That company will benefit from the assassination you are about to perform, and your stock in that company will greatly increase in value. Note: Do not sell your stocks right after the assassinations. Wait for three days after each assassination, and then sell them. You should make well over 75% profit off the stock. To make a lot of money, wait until you have accumulated a large amount of money at the end of the game before completing the assassination missions so you can earn a lot more in profits off the stocks.
To easily level up the strength and shooting skill stats to 100 in approximately ten minutes, use the following trick. First, buy a lot of pistol ammunition, and then find a bus. Jump on top of the bus, and stomp on it while it is moving. Make sure to move back and forth or you will fall off after stomping. Once your strength is at 100, use your pistol to shoot any nearby objects or people to increase your shooting skill to 100.
Keep in mind that, while these methods can help you earn money and improve your skills quickly, they may not always be legal or ethical in the game's virtual world. As always, use your discretion and play responsibly.