Consider why the other party is being difficult.

With an understanding of the other parties interests and development of a set of alternatives you may well be ready to start negotiations... but is the other party?
The other party may introduce o bstacles that stall the negotiation process simply because they aren't comfortable in beginning negotiations.
Other reasons such as the fear of losing control, feeling overwhelmed or perhaps resistance to a proposal or idea that they didn't come up with may also cause the negotiation process to get off to a slow start. 
By considering the ego and communication style of the other party and the key decision makers this situation can often be avoided. Remember t he more you know about a persons behaviour, motivation and communication style the better equipped you are to reach an agreement that meets both of your interests. 
William Ury believes that to reach agreement there is a need to build a “golden bridge” which draws your opponent in the direction you want him to move.  It is important to make it easy to reach agreement by involving the other party in devising a solution that is their idea, not just yours, and one which, satisfies their unmet interests and allows them to save face. 

Play the game.

Successful negotiation relies heavily on your own behaviour and communication style. These factors impact greatly on how another party perceives and interacts with you.
Many n egotiations can be lengthy and time consuming. An effective method of minimising the time taken for negotiation is to create an environment of participation. Involve the other party by asking them “How would you solve the problem so that we are both satisfied?” From there you can build on the discussion with ideas that are of interest to you or from ideas that you both have in common. If your opponent is reluctant to come up with ideas or suggestions and is holding up the negotiation you could try to offer alternatives for them to choose. This approach gets the other party involved and gives them an amount of ownership of the solution.
Often asking questions such as “What do you think will happen if we don't reach agreement?” will get the other party thinking about consequences if a solution isn't found and may help turn negotiations around. However be careful to avoid threats as they can turn the focus of the negotiation into a position of power rather than a solution.
Warnings are sometimes more effective  but use them carefully as they may provoke or cause the other party to react in some way.
In some circumstances introducing a neutral third party can assist in unlocking a difficult negotiation.

Its not over till its over.

Throughout the negotiation process try to stay calm until the end and not assume completion until you have both clearly understand the agreement. This may involve some clarification and repetition but it may just save you from future misunderstandings and may help to produce a lasting agreement.
A w ritten agreement is one of the best ways of securing negotiations and are more effective if they include guarantees, which aim to protect both parties from withdrawals or broken promises. Regardless of how you feel about the other party is it important to end the negotiation on a positive note. This will assist in creating a positive future working relationship.

References:

Getting Past No, Negotiating with difficult people: William Ury: Great Britain, 1991: Business Books Limited.

Getting to Yes, Negotiating an agreement without giving in: Roger Fisher & William Ur