What is your #1 tip for successfully negotiating prices with a customer?

When a customer says something like, Can I get a better price, never ever make an offer, even if you know your bottom dollar need.

Respond with something like, do you have an offer?  and when they come back with a number, start you emmy award winning performance.  Look concerned, puzzled, start counting under your breath, stare off in the distance, finally sigh and shake your head and couter offer higher.

sale made!