I've been trying to get my score up as well. The best way to get points is to get $$$. The health/armor/$250k cheat actually raises your criminal rating, the wanted level cheat lowers it by 30 points or so. I maxed out my money after getting 100% using the horse race betting (which is incredibly tedious) but only got to about 300,000 points. After you max out money that no longer raises your criminal rating. Even if you won another 120 mil on betting, you don't get anything. Also, if you spend money you lose criminal rating so if you plan to spend your money on weapons or something, don't bother.
Shooting down helicopters or hydras is the best way to raise your points after maxing out on money. Each heli/hydra gives you 30 points. So by that math I have to shoot down 23,333 helicopters or hydras. I just get on top of a building near a safehouse and shoot down helicopters till I get bored. You can usually get around 5 per minute, so it should take 77 hours give or take of nonstop shooting down helicopters to get the necessary points for King of SA.
If anybody has a better method of gaining criminal rating please let me know. Also, those people who actually got to 1,000,000, I salute you for being an outstanding time-waster!