This is all great advice, and similar to what I do.
It's important to first know how much you can afford to spend. Once you figure out your budget, remember you may have to pay additional costs such as taxes or shipping. Then, take that amount you're willing to spend, and create another amount significantly lower. This will give you a "range" of dollars you're prepared to spend. Ideally, you'll spend towards the low end of the range, but don't pass the upper end of the range.
Next is the fun part. I like to do some research on the web to see what a bass or a car (or whatever) is selling for. I look at all of the products that fall within that particular category and get a lay of the land. By lay of the land, I mean some manufacturers have certain features others don't and vice versa. It's largely marketing hype, so don't fall for it, but rather understand it. Sometimes that marketing hype creates a feature on a bass you later find you couldn't live without.
After you understand the marketplace, put the first two things together: budget and product range. Now you've got a strong idea of which you can target in your search and which you can blow off.
If you're buying from a retailer, there's almost always room to haggle. Some people like to talk about the "standard 30%" discount in the musical instrument industry. It's not bad information to have, but it doesn't have to drive your purchase price. Here's an example:
I string my Spector with DR Highbeams. When I first discovered these strings before I'd ever heard about the Internet (am I dating myself too much here?), the local dealer offered the strings for $27 / set. He showed me the MSRP which was well into the $50s, and said I was getting "much more than the standard 30% discount" because "I'm such a valued customer". Occasionally I'd go in there and his partner would ring me up for $25 / set and say that's almost his cost, so he couldn't go any lower. These two kooky guys would argue over the distributor's price increase or decrease, and how it would affect the consumer...endless, meaningless discussion in front of paying customer.
So, armed with the $25 amount that I'd happily scored such an amazing price, I saw some blowout sale GC had a month later for $24 / set. So, I went into GC, became friendly with the Accessories manager, and let him show my a MIM Jazz, a Sans Amp Bass Driver DI (still available for sale), and a Boss PH-3 Phase. He rang up the accessories and asked me if I needed anything else. I said, "Yeah...what are you getting for your DR Highbeams, 4-string set?" Are they really the $34 price you've got listed on the wall?
Two or three minutes pass watching him punch buttons on his keyboard, I discovered he was trying to figure out a trule good deal on strings. Then he replied, with a straight face, I can get them to you for $16 / set. When I heard that number, I nearly crapped in my pants and had to take 4 sets!
There's actually a point I'm trying to make...at least in this business, there is so much BS built in to the profit margin that if you're a sophisticated buyer, you can suck away some of that BS.
Keep it straight: know what you can afford, consider everything in your price range. Lastly, stick to your guns. Remember...many salespeople are on commission. If he can't get you to a price you agree on, he's losing commission to someone else. If he wants to be a success, he's going to work with you.
I'll also say this...in a consumer-centric society, it's valuable to treat these sales people kindly. If they feel like they're doing business with a friend, the psychology takes over the transaction, it'll be a done deal. Just make sure you stick to your guns.