Buoy Skipping: Intentionally skipping a buoy to keep your speed or to cut sharp turns. Skipping a buoy will stall your vehicle if you don't have enough boost (This does not apply to Normal and Hard, your vehicle will stall regardless of where your meter is). Fill up your meter with 1-2 tricks. Your meter should fill 1/3 to 1/2 after 1-2 tricks. Doing this will only lower your meter and not stall your vehicle.
Ramp Skipping: Intentionally skipping a ramp to keep your speed. This only applies to certain tracks, and can be tricky to preform. Some ramps have a gap on the sides that you can go through. The sweet spot can be a bit finicky, but skipping ramps allows you to keep your speed without taking the jump.
Hydroplaning: Probably the most important part of this game in general. By leaning back, you will go a lot faster than normally. Take advantage of every straight-away with hydroplaning.
Submarine: Diving underwater and lifting back up, gaining a small amount of air. This tech can help in a few areas, but I don't find use for it in every level. Using this, you can short hop over certain barriers and water obstacles.
Starting Hydroplane: Having a faster start acceleration by starting a hydroplane without tipping upwards. At the start of the race, lean back half way. As you accelerate, lean back fully into a hydroplane.