The thing is that you have to keep trying. I have completed several of both the Ampario surveys both the 150 and 450 ones but mostly the 150 ones. Some days I don't qualify for either but I have completed enough to buy a few clubs ans balls.
And don't forget the free trial offers you can get quite a few credits doing those as well. Just make sure on the ones that will charge you after the free trial to cancel before they bill you.
(Edit) I just made 600 credits taking 2 amapario surveys. It's not too often I am able to complete both in one day but it happens occasionally.