The cost of every car you buy is deducted to your balance. At the end of the game, the player with the most money wins, so choose your cars wisely! Do not buy as many cars as possible because it would cost you too much. Do not buy your cars too expensive as well unless you really, really want one. This is because the winners of the game usually buy only one or two cars.
Before auctioning, ask yourself if your speed cards match the color of the car. You don't have 5 or more speed lines matching this car? Perhaps this is not a good car for you. Even if you do have 5 or more speed lines, holding all 1, 2 or 3 is no good. You should at least hold three 4 or above speed lines. Holding lone lines and wild lines (especially if you have power unpredictable) is also very good.
Besides that, the powers are also important. Check out which powers match your style, and have this as a factor if there are 3 or more cars you do want to buy. Determined is better used with tracks with more straight lines, while aggressive is better used with more high-value cards.
When you play a card, you will most likely move the cars of the others. So do not hesitate to move them through long paths, do not hesitate to put them into position to be blocked next turn. Likewise, pay attention to where your cars are.
If you manage to be the first to reach a tight turn and block the other cars, then the other players will need to move your car in order to advance. And as speed lines are played from the quickest to the slowest, there are good chances that other players will be forced to play a good card on your car.
Remember that you are not obliged to bid on your cars. So if you see that another player took the lead, you can still bid on them to get a consolation prize. Likewise, if you manage to take the lead, even temporarily and that other players bid on you, they have a good interest in making you win! So they will play good cards on you in order to maximize their bidding payouts. Oh joy!