Step 1: Fill up the car with gas.
Step 2: As soon as you are done, open up the escape menu and create a save file (preferably a new one so you know which one to pick later).
Step 3: You have now made a save file where you have used your gas and earned the cash from it. This does mean you don't get infinite gas, so keep an eye on your amount. Now open up the save file, and the customer will still be there wanting more gas.
Step 4: Repeat steps 2 and 3.
Step 5: Refill gas when low.

The profit and fame gain from this might be best early in the game. since once later in the game you have most of the shops, they are probably faster combined.

Step 1: Pick up sand with Rudy.
Step 2: Get out of Rudy and run to the reset button.
Step 3: Reset Rudy, and now the sand is gone. :)

(Yes, I know you can turn off sandstorms in the settings, but this is also nice for dealing with the starting sand.)

I have noticed that if you close the warehouse gate while the delivery guy is driving out, the back door will stay open if he is under it. However, this is not something you have to worry about since the game still thinks the gates are closed, and therefore, you won't be robbed. If you are in a hurry and don't have time to wait for him to drive all the way out, just close the main gate, and you are good. Yes, I know there is also a way to turn this off in the settings, but what is the point of the game if you turn off every feature?