Perhaps by growing fast -- much faster than any turtle.
Cryptic colouration, body form, temporary arboreal habitat, and scuttling through the undergrowth all come to mind as possibilities as well.
Also, I think sauropods were well able to put up a fight against any predator up to around their own size, considering the thumb claws and the tail and all.
Are there any known juvenile sauropods complete enough to tell when the tail whip mechanisms or clubs formed?