6 Steps to Counter an Offer
1. Set your target price (your goal).
2. When calculating your target price use precise non-round numbers like, say, £437,500 rather than £438,000. It gives the number more credibility and weight.
3. Set your first offer at 65% of your target price.
4. Calculate three raises of decreasing increments (to 85%, 95% and 100%)
5. Use empathy, and different ways of saying “no” to get the other side to counter-offer before you increase your offer - don't negotiate against yourself.
6. On your final number throw in a non-monetary item (that they probably don’t want) to show that you are at your limit.
In this model you use all the "best" negotiation tools: reciprocity, anchors, loss-aversion etc. without having to think about them.