Look into a mirror and smile! Just before you enter a room for the first time or meet someone new, step into the bathroom and smile into the mirror. Speaking with a smile on your face will give your voice a positive tone, and will help to shift your mood if you are feeling down. While you are looking at your smile, take note of your general appearance. Make sure your clothing looks good, and that you have nothing out of place.
Squash your bad mood. If you are about to meet someone for the fist time, forget all of your troubles and be happy. Don't unload or "vent " on someone new. Whiners leave a poor impression on others in almost every way. Look for the silver lining in every situation, look at the glass as half full, and make lemons out of lemonade if needed... but always remain positive.
Make eye contact! Looking into the eyes of another is sometimes a difficult task for anyone, let alone someone you just met. When you meet someone for the first time, catch their eyesight for a few seconds, and smile when you speak. You will project a positive mood, and you will appear to be interested in the other person.