Manage your aerial crashes wisely: you only get 3 upward ones, and downward ones take some time to replenish. Use downward crashes to knock the guy into boosts, especially if you can get a special boost that way. At first the stop girl is irritating, but if you can get her special (by hitting her after hitting another boost girl without touching the ground in between), she'll give you the biggest boost of all. Youko (the blocking girl) can actually be more of a problem, especially if your angle is low.
Youko (the girl in the pink dress)'s special is made available by getting boosted by her, and becomes unavailable if you hit any other boost girl afterwards. Miki and Kiri's specials are available if one is next to the other. There are some other extra-special specials that have something to do with Youko, but I still haven't figured out what makes them appear.
My highest score is 5939.52m. What's your high score?