...so...I'm a negotiation adviser by profession.
There are lots of books that can teach you 'tactics' ("if they do X you do Y"). Tactics can be useful if/when the relationship isn't important, but what is far more powerful is getting some grounding in the fundamental dynamics present in any negotiation, and developing a broad repertoire of skills and experience that enable you to be strategic and nimble in-the-moment - in virtually any setting. There are a number of solid books in that vein: Getting to Yes (a classic in the field), Negotiation Genius, 3D Negotiator, and Getting Past No are all good places to start.
Since you mentioned some specific negotiations:
Cars: Information is power. Before big data & search engines, the most effective way to negotiate for a new/used car was to research the model & specs you wanted beforehand, then fax (yep, pre-email) the following message to 3-6 local area dealerships 3 days before the end of the month: "Hi. I will be purchasing X car with Y features in the next 72 hours. I have sent this fax to several dealerships, including yours, and I will purchase the car from whomever gives me the best offer for the exact model I have specified." A buyer was sure to get multiple offers because dealerships were eager to meet their monthly sales quotas. This approach also avoided the psychologically withering experience of dealing with a car salesman directly. Today, big data is eliminating the need even for the fax-based kind of approach. Truecar.com is one of the best sites available today to give you information on who's paying how much for the kind of car you want to purchase.
Furniture: I'm less familiar with what happens when you're buying furniture. Are you in stores? Buying off craigslist?
Cable company: A colleague of mine outlines his approach to lowering his cable bill here:
http://www.moneytalksnews.com/how-lower-your-bills-like-hostage-negotiator/By the way, the empirical research in the negotiation field is very consistent on this point: preparation is a key variable in negotiation success. The more quality time spent preparing, the better/higher outcomes negotiators get. In other words, don't think that by winging it you'll do as well as if you give it a good amount of thought and analysis beforehand.