Think about how you're sitting right now while reading this. Your shoulders are probably slouched forwards, and you're likely either leaning forward onto a table or backward into a chair. Now consider the fact that you are in this same position anytime you sit at a computer, on a couch, etc. and imagine what that is doing to your body.
The more you sit in a certain position, the more that position becomes 'natural'. Unfortunately, slouched shoulders and either a forward or backward lean in your back are NOT natural or healthy for your body. Constantly being in positions like these result in chronically tight chests and weak backs. Tight and weak are two things we do not want our muscles to be.
The good news is that you can (and should) combat these chronic problems. Step one starts right now, by constantly thinking about the way you are carrying yourself. Whether at your desk, on the couch, waiting for the bus, or anywhere else, you should always be considering your posture. Focus on sitting/standing up straight and pulling your shoulders back. This will likely cause some discomfort, but that's because your body believes your slouched position to be its new normal.
Step two involves exercises aimed at correcting this bad posture. There are a number of exercises that can help to correct this, but my favourite (and the one I've noticed the most results in clients from) is a shoulder press with a resistance band added to provide anterior/forward pull. What the band does in this exercise is pull the body into the mistake; shoulders slouching forward. This is important because it forces you to start actively fighting the bad posture which has been forced upon you by daily living.
Realizing the problem is easy, correcting it is ore difficult. But the sooner you start correcting how you sit and stand in everyday situations, the sooner your posture will start to improve and your body will thank you.