Once you get an offer you need to decide what to do. It's not very common these days to get offers that are full price. There is a considerable amount of skill involved in negotiating. This is the area where experience comes into play the most. There's really no way around it.
* You need to know what will work.
* When to use what strategy when to back off etc.
* You need to know the correct questions to ask.
* Listen to the tone of the voice as well as the answers.
* You really, really need to know your marketing statistic.
There is so much research available that there should be on excuse not to. For instance I write a blog for a national account that is geared just to market conditions in our area.
There are a lot of strategies that I use but they all depend on the situations and would be to difficult to cover here.