When working through negotiations and there is a difference in expectations, it is almost inevitable that one party will suggest "splitting the difference." This seems a logical step, and immediately creates an atmosphere of acquiescence or goodwill. This tactic can be quite effective because it creates an emotional appeal for the counterpart and it would seem, on the surface, that it would be selfish and non-conciliatory to refuse the offer. The problem is that concessions should not be made unless there is a trade of value. If you concede without asking for something in return, you are virtually saying, "alright, I asked for way too much, I was trying to gouge you, now I'll play fair and let's wrap this up." The result: it creates distrust as the negotiation continues or as future negotiations are brought to the table. Your counterpart will be expecting you to "ask for too much" upfront and will plan accordingly.
Instead, plan ahead and have equitable trades that allow for a concession. For example, when your counterparts requests you "split the difference," you can reply with a trade offer, such as, "sure, there may be a pathway to that... IF we split the difference in the price can we talk about a 40% increase in volume, or 30-day payment terms, or moving to a 3-year contract?" This approach not only opens up the door for more trading, but gives your counterpart some options. Remember, a very important part of this strategy is to be prepared for those "asks" and to have ready-made trades in mind when the "ask" comes.