See if you can practice staring but be aware of your face at the same time and make it correspond to your own thoughts and feelings, rather than the other person? Tilt your head and frown slightly if you have a question or don't agree with something they say, smile and give a little "humph" with your shoulders if something is mildly amusing, raise your eyebrows or widen your eyes if what they say surprises or interests you. In other words, pay attention to yourself and how you are feeling about what is coming in and make it correspond and give the other some silent feedback.

When you are talking, occasionally ask an open-ended question (a question that cannot be answered with just a "yes" or "no"); ask about the person. I practice with grocery store clerks and will ask them how many more hours they have until they get off or until their break (especially if I see them sigh or yawn :-) or if the weather is doing something outside I'll comment on that and tell them they are lucky/it's too bad they're inside.

Start with conversation about things anyone has to know about (weather, their own activities) but don't grill the other person, give them something about yourself to help guide them; "How has your day been so far? I just got up; today is my day off and I'm meeting a friend for lunch after I finish here!" That shows you aren't just greeting them ("How are you?"/"Fine.") but want a little conversation. Then, you listen to their answer and pick something up from there that has you curious and comment on it or ask another question, etc.

For example, if I have the "How has your day been so far?" with the grocery clerk and they say, "Not many shoppers in here so far today, it's been pretty light" then you can rejoin with, "I guess Tuesday morning would be pretty light normally? What day(s) are busier or lightest? I'm sure the weekends are busy with everyone off work."

When we were having a heat wave last summer, I lucked out and got a clerk and the conversation was about how her apartment did not have air conditioning and her poor parakeet was stuck in that warm weather but we decided it was a tropical bird (and I confirmed for myself that she had made sure it had enough water) so okay but then went on to talk about after work and how she had just that moment during the conversation decided that she'd have her boyfriend pick her up and they'd go somewhere with AC rather than to her apartment, etc. It was a great, interesting, interpersonal conversation, beats just standing there watching the clerk scan the grocery items?