Contracts. Contracts state that you are giving up certain rights (such as North American
Negotiating prices. You receive your contract and the payment is $25 less than you thought it would be. Do you email the editor and complain? This depends on how bureaucratic the magazine is. If the editor always responds within 24 hours to your emails and seems eager to work with you, you can bring up the price. However, this is best done before a contract is brought up. These are almost impossible to change if you're working with a major mag.
Negotiating rights. If you're unhappy with the rights you're supposed to be giving away, talk to the editor. Be prepared to wait, be prepared to be frustrated, make sure you know what is important and why it is important. That said, I relax my rights-worries when dealing with big name companies and magazines with good reputations. Be worried when dealing with Small Zine Nobody's Heard Of, and comb that dang contract.