i got like 200 million without even trying, i got him up to 100,000 feet and just made him going full force to the right. after that i just left him there for like and hour...
After a certain point, it's all about patience. I've gotten the hedgehog to above 110,000 feet in free play; if you have the highest level fuel tank and fuel siphon, you can pretty much bounce forever. At a certain altitude, you can even leave the computer alone for a few of minutes and come back to it without having hit the ground.
Jumping off of Sterling7's comment, I think it would be cool to know what everyone's best run time is? I went for 12 minutes in one run...and then got really bored so I landed as fast as I could.