1st of all, Make sure you have a gun(reer London, screw drivers and sharpened pencils wont' cut it)
2nd of all, shoot the person before he shoot you( you can always argue that it was self defense, especially if the guy had a gun.
Don't start unnecessary fights, unless it involves money and hiding evidence
Make a no go zone bloke in your neighborhood, and let other gangs know that they can't trespass that zone
Good luck guys
