A smile is a gesture that signals your desire to make friendly contact. So, yes, smiling does help. It shouldn't be constant, though; smile when you meet someone or are introduced to them, and smile when you are saying or hearing something that's funny enough to notice but not funny enough to laugh at. Also smile when you hear good news from someone.
Most people, when you smile at them, will smile back. It means, "I acknowledge your presence and also desire to make friendly contact." In some contexts, such as meeting a familiar person in a hallway, an exchange of smiles can be a form of greeting, and does not necessarily mean that they want to stop and talk. On the other hand, if they stop and face you, they probably want to greet you.
Perhaps, if you don't smile at people that you want to talk to, they are misreading your body language as saying, "I want to be alone".