Firstly, you'll need to find a car that's quite nippy. Anything that doesn't look like something your mum would choose is fine, just make sure it isn't an open-top or you'll get your face shot off before you've even pulled away.
Now, head up to any of the bridges that would lead you to the next segment of the city (either of the ones highlighted in green here are ideal). As usual, the game's bridges areclosed off to begin with, so even attempting to pass the police barricade gets you an automatic 6-star wanted level. Considering it's virtually impossible to get 6 stars by any other method at the start of the game, this is perfect for our needs.
Above: Just walk past the cars - you'll have a six-star wanted level immediately
Drive your car up to the barricade and park it with the driver's door close to you and the car ready to pull away at short notice. Then get out and venture forward past the police. As soon as they start shooting you and the six stars appear in the top-right corner, peg it back to your car and get in. Now drive!
The trick now is to simply keep to the highways. With your speed up, the cars ahead of you can't turn and chase in time and those following will have a job keeping up. So, as long as your driving skills are good enough to keep it pointing the right way (easier said than done at these speeds) and you can avoid the cars that come towards you, you'll be fine. Oh, and don't worry about the army getting involved - they don't. Yeah, we were disappointed too.