Make eye contact, fairly consistently throughout conversation. That being said, the natural length of time for constant eye contact is 6(yes SIX) seconds. Every six seconds you should blink, glance quickly at another part of their face, but at least blink and let your eyes become unfocused. NOT doing this will make you seem creepy, and will make you feel particularly awkward. Remember to blink. If you are uncomfortable with making direct eye contact is looking at the persons nose, between their eyes, it appears as though you are making eye contact but you are not and it can be easier. Remember to blink, glance away, ect every six seconds even if you don't feel like you need to(this will make it seem less awkward looking at the nose) but they don't know thats what you are doing so it will make them feel uncomfortable if you don't.
Other things to keep in mind:
Body language is key. looking someone in the eyes is respectful and strong, but looking for too long is dominant and aggressive. Not looking someone in the eyes at all or minimally is submissive, shy and displays a lack of interest. Looking away more frequently than every 6 or for a longer period of time then about a half to quarter second will make it clear you wish to end conversation. Your chest, secondly is also an important part of your body people often overlook when it comes to social etiquette, and can be a real tell tale sign that you can use to judge peoples interest levels and they wont even know it. Simply put, position your chest in the direction of the person you are talking to. Its kind of hard to describe but you want your shoulders and heart to point to the thing you wish to show interest in. Its subliminal but it matters. For example if you are making eye contact but your chest and upper body is pointing towards someone else that is a clear sign that you find that person/situation more interesting or desirable. Body pointed towards speaker shows interest, Body pointed elsewhere shows disinterest or multiple points of interest. You never want your muscles tensed- this shows a fight or flight response and people will automatically back down and avoid in an attempt to stop this response, which may make you feel more uncomfortable in the long run.
Um, hope this helps some. Don't concentrate on doing all ofthis all the time, it will make you seem awkward, just every once and a while quick check the signals you and others are giving off.