Buying a sofa is a major investment, and there is no reason you shouldn't be able to enjoy what you buy. No one needs the frustration and shopper's regret that comes from a hasty and ill thought out purchase. A good salesperson will understand and help you find the one that is best suited to your needs.
The most important things to consider are quality and comfort, especially if you plan to spend any amount of time on your sofa.
The best looking sofa in the whole world cannot measure up to the most comfortable one. That is why you need to test drive one before you buy, just the way you would test drive a car.
Take Your Time
Give yourself plenty of time when you are trying out a sofa so that you get to know it. A mere few seconds of quickly plopping down on one is not going to tell you much about it. A good salesperson will understand and not bother you while you are trying on different sofas for size. You need to get an understanding of how it feels when you stay sitting on it for a while.
Test Different Styles
Test many different styles, and make sure to try them out in a way you would use a sofa at home. Do you like to sit with your legs curled up? Do you like to lie down on it? Do you feel happiest sitting in the center?
Consider the height and depth of the seat. For taller people, it is best to have deeper seats.
If you find it hard to rise from a seated position, try a shallower seat.
When you sit down on a sofa to try it out, take into account the height and feel of the back as well. Do you prefer a tight-back sofa or one with loose cushions? Try out all different cushion configurations to see which one feels right.
Do remember to take your shoes off before you place your feet on a sofa in a furniture store if that is your sitting style.
Test Different Fills
Every person differs in what they consider to be comfortable. Some would like a sofa they can sink into, while others prefer a firmer seat. Which one do you like? Try sitting on different fills to select the level of firmness or softness that you like.
Consider if you want the same level of softness or firmness in the seat and back. They do not have to be the same.
Test Different Fabrics
Each fabric is going to feel different, and it will add in a very different way to the overall feel of a sofa. You may like the look of heavily textured fabrics, but do you like the feel of them? Does a surface feel too slick? Do you like the sound leather makes when you shift your weight on it?
Sit on different fabrics long enough till you find one that is right for you. This is especially important if it is going to be "the sofa," that special one that is used all the time.
When testing for fabric, you should also check to see if it wrinkles easily or stretches. If it wrinkles easily, it will be difficult to maintain its looks for a long time. Sagging fabric will make it look dilapidated very quickly.