I've not talked a lot about contracts before now because I'm not a contract lawyer and I'm not qualified to give legal advice. Fortunately, Passive Guy is a retired contract lawyer and, while he no longer gives individual advice, he does keep the rest of us informed over at his blog, The Passive Voice.
I'd like to direct your attention to a post PG made yesterday regarding a trend in publishing contracts: the non-compete clause. This tiny clause can mean a world of pain for you, oh intrepid author, so be forewarned.
I've fortunately not run into this yet, as short story contracts don't get quite that ambitious. But whenever you find contract language you don't understand, you should consult an expert. Don't be afraid to propose changes to contracts; just be aware that the party who is most willing to walk away is the one with the better bargaining position. I recommend reading Richard Curtis's How to Be Your Own Literary Agent, and thinking about what contract issues are non-negotiable, and what issues you're willing to compromise on. Thinking about this ahead of time will make it less likely that you'll just sign on the line in blissful glee when that first contract crosses your desk.
Remember, an advance is fleeting, but a deep-seated contract screwing can be forever.