Negotiating really means compromising. Sometimes you get everything you want and more, and therefore you don't have to compromise. But successful negotiators know they have to be ready to give something up--even if they never actually have to do so.
When your position is weak, you may have to give up more than you would like. You may feel that you will not achieve much by any negotiating effort, but that doesn't mean you should not try. Your negotiating efforts are successful when you have to give up less than you thought you would or you obtain more than you thought you could. Experienced negotiators know that their efforts obtain profitable results even when their position is weak.
Know what you want. Establish a list of minimum requirements that are acceptable and another list of what you would like to have. Then compare the two lists and rate the items in the order of importance.
In other words decide on what you are most willing to give up without sacrificing any of your minimum requirements. Understand that what is most important to you is not necessarily what is important to the seller. The seller may or may not want the things you think he does. Make no assumptions about this. For example, it is easy to assume that the seller is most concerned about maximizing price. This is no more true than it is to assume that the buyer is always most concerned about price. In some cases, the buyer may be more interested in getting early delivery. Likewise, in some cases, the seller may be more interested in getting an order to increase sales within a certain sales period.
This is not to say that either buyer or seller is not concerned at all about price or some other aspect of the sale. It simply means they may be less concerned than they are about other items.