When you see rent quoted on Craigslist, in the paper or on a community message board, it's just that: a quote. This is the landlord equivalent of a buy-it-now price on eBay -- the top end of what you could pay for the space.
And like on eBay, whether or not you pay the buy-it-now price depends greatly on supply and demand. Simply, how much housing is available and how many people want it? Every month an apartment sits vacant costs the landlord a significant amount of money, so by recognizing a rental without a long line of prospects lining up at the door, you immediately bargain from a position of power. Consider making an offer at the end of a month, with the ability to move in immediately. On the flip side, if you're renegotiating an existing lease, do so well before the lease is up so that both you and your landlord know that you have time to find another place if negotiations don't go your way.
Then consider the fact that by stating a price, the landlord offers this price to anyone who meets the rental criteria. If you have references showing that you're a clean, responsible renter, that may be worth a cost reduction.