There are currently 10 Agency Vehicles and 6 different Boost Bonuses. When a Boost Bonus is activated, the boost meter of the vehicle is refilled significantly faster than normal. In order to activate the particular Boost Bonus of the car you're in, you must perform certain actions while inside the car.
When using keyboard/mouse, the default button for drifting is [Spacebar] and (A) for XBox/Steam controllers. When approaching an intersection or sharp turn, hold the drift button before you reach it, and then start turning in the direction you want to turn in.
Cars With Drifting Bonus
This is probably the most straight-forward Boost Bonus. Find a ramp or a small hill, and go up it with speed; the longer you're in the air, the more of your boost meter refills. This bonus is also a noticeably fast refill, this is likely due to jumps not lasting long, and being somewhat scattered, especially in city driving.
Cars With Air Time Bonus
This is probably one of the more risky bonuses to achieve, as it requires the player to drive along roads on the wrong side, into oncoming traffic. However, if the player is able to see far enough ahead on the road, this Bonus can be quite easy to maintain.
Cars With Oncoming Lane Bonus
Another simple Bonus to achieve: Go. Fast. Any time the vehicle is moving at or above 100 km/h, the boost meter will be filled faster. So simply use your boost to go faster, then, while at that fast speed, rebuild your boost faster, so you can go faster, sooner (the person writing this confused them self while writing that bit).
Cars With Speeding Bonus
Another risky bonus, but can also be quite lucrative at the same time. As the name implies, the player must get near other vehicles, and pass them closely, but without touching them. Unlike the other Boost Bonuses so far, this one can refill part of the meter while the boost is being used; so go ahead and speed along the highway with unlimited boost. Additionally, the meter is refilled in 'chunks', rather than an increase of the regular refill speed. This Boost Bonus applies regardless of whether the car being passed is stopped, going in the same direction as the player, or the opposite direction.
Cars With Near Miss Bonus
Similar to Near Miss, Vehicle Kill refills the boost bar in 'chunks'. This bonus requires lots and lots of destruction in order to maintain boost. ram into any car you can see -- legion or civilian -- and if it explodes, you get to speed away from the destruction in awesome fashion.
Cars With Vehicle Kill Bonus