Here is a fun career tip: smile often.
When you are just starting out in your career there is a constant flow of new things to learn. As you are absorbing those new things your brain can get pretty busy. When your brain is very busy sometimes you forget to smile. By being aware that smiling is good for your career you can remind yourself to smile. It is a pretty simple idea, but it works.
People like to feel good. When you smile at someone you make them feel good. When you frown at them they feel bad. Often times you do not realize what facial expression you even have on your face, let alone how that translates to someone else. A basic awareness gets you more than 50% of the way there.
As you progress in your career you take on a leadership role. Regardless of your title, people will look to you since you have more experience. They will read your facial expressions even more than at the start of your career. Smiling can really help.
If you are walking through your office and you make simple eye contact with people and smile, people will feel at ease. If you are not aware of your facial expression and are very busy, chances are that you may come across the wrong way and leave people feeling a bit uneasy. Here again, being aware of that fact is more than half the way there.
I am not suggesting that you have to fake being in a good mood when you may not be. My point is simply that your facial expression says a lot more to people than most of us recognize on a daily basis. If that is the case, then you might as well make it a positive advantage for yourself.
I have heard that it takes just a few muscles to make a smile, but that it takes over twenty muscles to make a frown. I have also heard that when you smile it instructs your brain to release good chemicals that are associated with feeling well. So smiling is easier than frowning, and it makes you feel good. Bonus.