Ok, here is my guide:
1. Participate in all events. In one night of events I normally win at least 20k
2. Do not buy drugs
3. Do not break the law or you lose loads
4. Find an unowned car and check the value. For instance, I once found an unowned ZR-350 and I sold that for quite a lot.
5. If someone is selling a car, buy it and then resell it for more
6. Do not buy guns. They are useless. If you kill someone, you get suspected, which leads me back to number 3...
If you play as a cop, you get guns for free, but don't try abusing the /duty script for the guns...
7. Join an organisation. ALS for instance, helps to make money because as a corporation they are generally more respected for their limo driving work than other people that steal random limos and drive people to places. ALS I think sometimes rewards some members with a huge amount of cash to buy a limo, a house or a bussiness. Joining an organsiation has a whole bunch of advantages but please don't just join for the cash...
8. If you are a driver, paint your car in a strange and noticeable colour. That way, your car will be chosen above everyone else. I own a limo but it spawns in pink (126) so people are more likely to get in my limo than most people.
9. Do not deal drugs often. I have never done this at all because I am a cop. Also, I have noticed that many people that buy drugs are undercover cops who are obtaining evidence and can lead to you being arrested or sued for thousands.
10. Do not sell modified cars. This can be tempting as you see people spending 2k on car mods and then selling the car for 4k and making a lot of profits. DO NOT DO THIS. Here is why you shouldn't do this:
1. Server restarts cause you to lose the modified cars and money spent on them
2. Some people do not have enough cash for the mods on the cars
3. Everyone knows that /delivercar does not save modifications
4. The ARPD arrest illegal car dealers and impound their vehicles
5. The most obvious reason...and possibly the funniest for me...OWNERSHIP. If you steal somebody's car, mod it and then attempt to sell it, the owner has the legal right to take their car back off you at any time...meaning that they end up taking away your cash as they are covered in mods. This has happened to me. A while ago, when I owned a remington, at least once a week some guys in GSF would steal my car and fully tune it. I would do /heydude, find my car in East LS or in Grove Street, take my car back and then I would have loads of free mods
