Learn the business you are in (or entering).
Protect your work – be it writing, photography, illustration, Art. Learn contracts and copyright. Do not sign anything that you don’t 100% understand.
Don't take anybody's word for it, like “yeah, the contract gives us permission to do X but we would NEVER do that”. Even if that might be true for some, people change and contracts stay. It is very hard, if not impossible, to change contracts after the fact.
We all have choices, and the responsibility for our careers lies 100% on us. Don't blame anyone else for the mistakes you make.
(UPDATE: a longer blog post about this topic will follow)