Some people enjoy the bonus zone, but I ignored those.
Instead, I just made my way to whatever freeway was in the Insurance Fraud area and patiently waited for the first cars to spawn.
Don't look around too much, as the refresh seems to mess with you and it takes longer to get a car to spawn.
Okay, now when you see a car, or a big truck or any likely candidate...run towards them and JUMP just before they hit you....make sure that when you do get hit you are pressing one of the triggers.....but now that you are in the air a bit, practice controlling your fall (I held A to slow my descent and get better control).
The real combos come if you can control your fall into either the same vehicle that hit you or another one. Play around with it a bit until you get the knack.
Before too long, you'll get your adrenaline boost and be able to use the same technique for even better combos.
Using this method, I was able to get a combo over 170K alone. A friend hadn't gotten the Achievement, so I offered to co-op with him and showed him the ropes. He was able to get over 230K in a combo, handily beating my best.
....AND we got him the Achievement and the in-game bonuses.
Okay, if you don't want to head to a freeway, I would then say go for the bonus zone, but do everything like I said. Don't look around because you will slow the spawn rate for cars, and then jump/trigger/A button/control/combo.
Keep at it, and you'll get the knack of it. Good luck.
I like to make mimes angry by playing blank CDs at full volume.
I guess its a matter of preference but the Bonus Zones certainly increase your adrenaline (spelling?) much faster. I have been able to beat all of the insurance fraud levels within about half of the given time. Just make sure you control your character in the air and hit multiple cars. The freeway tip is a good one - the trucks generally give you about 10 - 15k in just a single collision.
These are little tips i have used: Find a spot where the cars are coming uphill at you, this way you will get thrown way in the air. Always try to stay in front of the first car that hits you if you can - this way you can get multiple hits without having to find multiple cars. There were 2 others i was going to post - but i forgot now, i will post if i remember.