1. Though there is little difference, building a car with more bricks will make it heavier with a higher top speed and slower acceleration, and with less bricks, vice versa.
2. Following on from this, brake when using the grappling hook on an opponent to not only stop them in their tracks, but also drag them backwards whilst you are pulled forwards. This is especially useful in one-on-one races/scenarios. Having a heavier car and turning at about 70 degrees to the rope (much further and it will snap) pulls the opponent back faster.
3. If you are parallel to, but slightly further ahead than, an opponent, drive into their side to drastically reduce their speed. Great for encounters with CPUs with similar/slightly faster speeds than you.
4. Use Mummy's Curses on narrow sections of track to annoy the majority of your pursuers...
5. On Alien Rally Asteroid, be sure to grab the shield at the start of the track - you'll avoid being slowed down by the UFO and your opponents will take their time catching up with you.
6. Don't bother using the warp shortcut on Rocket City Run - the warp isn't as long as regular ones and thus you'd do better to get a green brick before the warp and three white bricks when driving past it.
7. For a harder game experience, avoid using green bricks, except when facing Rocket Racer and Veronica Voltage... unless you think you're skilled enough?
EDIT: Regarding the speed/car design debate, when I was experimenting with the grapple trick, a car with fewer bricks was definitely less effective, and this was with the same opponent each time (Redbeard). That, and the game designers mentioned in the developer's journal that realistic physics were added to custom cars. Mine's the 2001 version.