After you complete all 21 missions on a map, one gang member from each gang will close in on your location and try and kill you and often they'll pack powerful weapons such as a rocket launcher or a flamethrower, so be very careful. You'll get a warning at the bottom of the screen and the gang members will arrive in one of their gang cars. After you kill all 3, or they kill themselves, the entire map will errupt into chaos. Pedestrians will go crazy and start attacking everyone and pulling people out of cars. It's very easy to be carjacked and/or killed in these situations. The flamethrower is a good weapon to have equipped in this scenario.
The flamethrower will kill anyone it touches. It will slowly drain health and once on fire, it can't be put out and the person will die, that includes you. But because of this, don't bother wasting 15 uzi rounds into SWAT members and FBI, just set them on fire. However, if you anger the Hare Krishna or Scientist gang to the maximum, their gang members will carry flamethrowers so be very careful when in their territory. Having a flamethrower about your person at all times is a good idea.
The electro gun has a fairly wide range, and can attack people on the other sides of cars. So taking cover behind a blown up vehicle and attacking oncomers with the electro gun can save yourself some valuable health/armour.
Each gang payphone will have health and armour so if you need either or both then head to the nearest payphone and collect it.
If you have 5 bars or maximum respect with a gang, any cops who are chasing you will be shot at by the gang members with pistols. The SWAT teams ans FBI will slaughter your gang members, but they do provide useful cover so if you're in trouble, head for the gang territory. When you resume a game, you need to do something to alter the respect level before the gang will protect you, even if your respect is maxed out. Running one of them down in a car is a relatively painless way to do it.
Gunning down gang members will alter respect far quicker than if you do it in a car, presumably because you're partly hidden in a car. So therefore, if you need to alter respect levels quickly, do it on foot rather than in a car.
On Mad Island in the Industrial section you'll encounter a nameless gang, many of which carry pistols. They'll shoot at you regardless of your respect levels with the other gangs, so if you need to head into there for any reason, be prepared to brave a lot of pistol gunfire.
Even though SWAT Vans and FBI Cars will chase you at the speed of a Cop Car, when you get into them the SWAT Van has the speed of a Bank Van and the FBI Car has the speed of a regular civilian vehicle, so if you need maximum speed a regular Cop Car will have to do.