Snaking

Easy enough, hold R and swivel the left stick back and fourth at a decent rate. Don't go too slow, or you'll slide out of control. Don't go too fast or you won't get any sparks out of this. Release R as soon as you see blue sparks form at the wheels.

Turning around on a dime

Hold R, reverse, and hold either left or right. This will let you turn around in much tighter spaces, along with also being useful for optimization.

Preserving boosts in the air

When drifting off of a ledge, your boost will continue to charge even when you're off the ground. Just make sure to keep holding R and a direction on the left stick.

Extra tip: when the camera is panning back to your car after getting a relic, you can still move, so take that into consideration when trying to optimize segments.

If I find anything else of interest I'll update this guide.