Long one, but based on my experience:

I find the best way to barter is to make a good connection to the seller. Be friendly and self-assured. Ask questions about the person, his company or the product. Change the subject sometimes when it can, this broadens the discussion.

If the room permits it walk around, the seller will then follow you, creating a dependency in the mind of the seller, he has to follow you.

After an initial connection was made, break contact somewhat. Start looking at other stuff in the immediate area. Be a little uninterested. This makes the seller start working for your appreciation.

When you are satisfied with the initial contact and the follow up, ask for the price. Show that you find this expensive, non verbally (raise eyebrows, look disappointed, etc.)

Change the subject again, let the price drift out of the conversation. When you finish this line of conversation, bring up the price again and logically explain why the price that was offered is too high. This can be too high, just in your opinion, too high compared to the competition, or anything that sounds reasonable.

Do this until the seller asks you for what you think is a reasonable price. When this happens, you are in control of the price discussion.

Pretend to (or actually do) think very hard on this, make reasonable assumptions, assure the seller that you find this interaction important (this makes them happy as you were dismissive earlier). Come up with a price slightly below reasonable.

From there work your way to a middle ground. Make the haggling fun and interesting by showing interest in the person of the seller and different topics that may come to mind. Finally, make your final bid. At this point, lose interest in the person. The seller will most often say something along the lines of "i need to think about this", if not, suggest that they do this. Now start doing something else. Go to the toilet, sit in a different car, etc. I actually start reading a magazine sometimes when i am at this stage. Usually, the offer that comes now is good enough. Now you can start haggling over details, such as services, a radio in the car or whatever if you want.

make sure you show appreciation on the process when you get to an agreement. Positively reinforce the sales person and you will both be happy with the sale.

In general, follow your gut feeling. Be friendly and walk away if you are not satisfied.

TL:DR: Make the seller work for you.