Every negotiation is different. Price is what most people think negotiating is all about, but that is rarely the complete story. If a seller has found their dream home and has bought contingent to the sale of their own, or if they have been transferred in their job, a fast sale may be just as important as getting top dollar on their property. Conversely, the seller may want to retain possession while waiting for a home they have purchased to close. Finding a buyer who will rent them the property during their other escrow process may be important enough to deduct several thousand off the purchase price. 

There is no trick in negotiating.  You need to know what you absolutely have to have from the transaction, and what you are willing to give.  If the property needs repairs the seller cannot afford to pay for before closing, a buyer willing to negotiate this point may be able to get a lower price or some other concession.  If the seller needs a fast sale or to retain the property after closing as a short term rental, this is an opportunity for a buyer to get a lower price or some other concession.  

Many times these opportunities go to waste.  The reason is simple; the agents don't communicate.  Make certain your agent is asking all the right questions.  A concession that is meaningless to you may be priceless to the seller.