Effective Negotiating regardless of Chinese or English

so after being happy with myself over being able to ask the shifu if he could replace the battery in my watch (he could), and then getting that done, I probably overpaid a bit for the service. Well, maybe a lot.  Like any American not used to negotiating, and being in a fancy department store where we're not conditioned to negotiate, I probably was not in the frame of mind to negotiate, but should have been.

I'm curious if anyone out there has specific strategies on how to bargain to get what you want.  I was aware of the Best Alternative to No Agreement (BATNA), and I probably should have had a position prior to getting the battery replaced, but again, it wasn't clear that this was the place to negotiate.  It wasn't a street corner, and it wasn't a sidewalk; this was a department store with a little side area where a vendor sold batteries and watch parts.

It's also easy to know that on the street with some dude coming up to you and trying to sell you something that you can negotiate with him, but the line is grayer around a department store. Thoughts?