The trick I've always used (this worked in SR2 as well as SR3) is as follows:
- Start the mission
- Get to the nearest highway
- Stand in the shoulder until you see a car coming (ideally a long line of cars, or a large truck)
- When the car is getting close, run into the street and towards the hood of the car.
- Jump and hit the ragdoll button (in SR3, don't use your "sprint" for this, or you'll do a dive roll instead of a jump)
- If there are more cars behind the one you hit, try to aim yourself at another car while in midair.
When your adrenaline maxes out, try to "surf" down the highway by hitting the front of a large truck, and then guide yourself in midair so that you hit it again as you're coming down.
Cars appear to be more plentiful and drive faster on the highway, although this has been adjusted in SR3, so smaller streets are a more viable option. If you're in the road, they may slow down (this was much worse in SR2), which reduces the chances they will hit you hard enough to give good cash. Thus, stay out of the road unless you're just about to get hit.
Any road that has a low overhang (some of the between island bridges, for instance) are particularly bad for doing this. Your height will be severely limited. If you find yourself in an area with a low ceiling, do what you can to get out from underneath it.
In general, the more cars you hit in one fall, the better. Each car hit doubles your score for the hit, as well as adding a decent amount to the money counter. The cars don't have to be unique; you can bounce back and forth between 2 cars, and it will continue to count towards your multiplier.
If you're in adrenaline mode and the meter is running low, you can recharge your adrenaline if you "finish" a large hit before you run out. Adrenaline mode also doubles your score for any hits that finish while its active, so finishing a hit before it runs out is better than landing afterwards, if you have a choice.
Finally, if you're playing with a partner, one of you may want to hang out in the "extra adrenaline" zones. The highway is a much better place to get massive hits, but having your adrenaline maxed out is enough of a benefit to make it worthwhile if you have two people.