The TVs you see in the store are run in Vivid mode. What Vivid mode does is it turns up the color settings, brightness, contrast and backlight really high. This makes the picture really bright.
Now you might ask why your TV at home doesn't look super clear when it is in Vivid mode. That's because Vivid only looks good when the enviroment has very bright flourescent lighting(like the ones in store) that let the colors pop off the screen more.
Another reason is what they are showing on the TV. What you see playing on the TV is blu-ray movies with high bit rate and super clear clarity. There are content that are designed specifically to show off the clear picture of the TV to impress the buyer.
If you're watching cable on your TV, it will most likely be low bit rate unless it is a channel that is for specifically HD. It won't look as clear than the TVs you see in the store. At home you should be using Normal or Movie mode if your room has a natural lighting and not super bright like the ones in the store. It will make the picture look more natural but won't be like super clear like the ones in the store.
If you want your TV to be as clear as in the ones in the stores, you're going to need an HDTV.
A Blu-ray DVD Player so you can watch blu-ray movies. Which is the best looking movie format.
And if you want the ultimate theatre experience, you need a Home Theater Speaker Package for great sound.