Big Blue: Drift Highway
Boosting needs to be done early on, in the tunnelled sections under the water and the wide right hand corner that leads to the first drop off point. Watch out for the nasty chicane on the second to lanst corner, as it can really mess you up. Hug all the corners as tightly as possible. Don't bother trying to take out other vehicles as this will only slow you down, you can do it on the next course. The course is a lot harder game's own two-star rating implies.
Port Town: Aero Dive
Although initially an exiting track, this will be nothing more than a formality come master mode. If you found you bombed on Big Blue, this is the perfect track to take out the racers occupying the top few positions on the first lap. If you don't get a certain vehicle, don't bother trying to get them just win the race. You can gain positions easily, especially on the downhill section after the big jump. use most of your boost on that.
Green Plant: Mobius Ring
Use the first lap to take out any threats to you campaign, because it will be a struggle on the two laps that follow. You should find that you can boost pretty much the whole way around it. The only danger is uneveness. Give the track's uppermost sloping sections a wide berth, and it just takes an attack on you wing to send you off.
Port Town: Long Pipe
Due to the nature of this track the only killzone is on the final straight. As a result of this, you need to make absolutely certain that you win. There are a couple of problems on this course. The first is disorientation. You get thrown around violently on parts of this course so make sure you're ready for it. The other one is the big fan rooms. There are three of these on each lap to contend with. If you hit them they will stop you dead, so make damn sure you know where they are and how to avoid them. Hitting all the boosts here and using your own sensibly will see you win easily.
Mute City: Serial Gaps
Personally, I find this the second hardest track in the game (Phantom Road is the hardest). I've never managed to win this race on Master mode, 7th is my best. Hit the jump pad at the front half way through the lap. You'll need to be boosting once every 3 seconds, and try to land on the floor from the big jump as the bottom one loses you three or four seconds. Before the race look to see how far in front you are of the rival so you'll have some idea where you need to finish. Pause it just before you cross the line to see where you are. If it's not good enough then retry. If your not sure then choose whether to take the risk or not. One last thing: on the rise that leads up to the final straight, don't boost at the apex, otherwise there is a chance that you'll fly off the track, with no chance of getting back on. This is easy to do on the last lap with your boosting finger working faster than your brain!