What hidden charges/fees/upgrades can I look out for when signing a contract for a used car?

This is my first time buying a (used) car at a dealership. I know about negotiation and have researched this, and feel somewhat comfortable in negotiating a low MSRP. BUt, I am concerned about what hidden fees they will charge that are not necessary to boost up the price. Specifically, what can I look out for> Thanks!