For keeping up speed for a long period of time, go on the freeway. You've got a lot more room, less twists and turns, and clipping another car won't lose you that much speed if you're lucky.
Also, bare in mind that if you have to stay above 150mph, it doesn't mean you have go top speed (e.g. 200mph). All that'll do is make you eat up more road quicker, and make it harder to dodge traffic. So if you find yourself doing 170/180, ease up a bit.
Lastly, two obvious points. Use a fast car (e.g. McLaren or Lambo), and stick to the correct side of the road. Don't try it in oncoming traffic.