For those who do not want to grind out the standing by doing either grinds or races, I have found another method to boost standing.
In the Southwest corner of the map (map, below) there is a truck with two rails in the bed. If you ride your K-Drive between these rails, you "float" and your combo does not end.https://www.trueachievements.com/gamescreenshot.aspx?gsid=17...
I have found the best way to increase your score is to jump as high as possible and land between the rails on the side that is farthest away from the truck. Then you increase your multiplier by using LT and A, as shown, below. Your K-Drive will slowly move toward the truck on its own. Once you are close to the truck bed, stop touching the controller and you will land in the truck, completing the combo.
Here is a clip of me doing it: