Research, homework… call it what you will. Knowing what you’re doing before you march into a store is one of the biggest shopping tools you can have.
When buying large items, it really pays off to do your research first. Consider makes, models, different manufacturers, what sort of options you want, size… right on down to the color you’d prefer. This way you’ll get what you want at the price you’re willing to pay, and can easily brush off an aggressive salesperson’s attempts to push you towards you don’t want (or can’t afford).