[lutorm]

Looking someone continuously in the eyes can also be judged too intense and uncomfortable, and the most natural reaction, at least to me, is to lower your gaze momentarily. This is the thing that (to me, at least) seems least like I'm bored or checking out of the conversation.

It is unfortunate that when talking to females, this puts your eyes in the region of their chest, and there have been instances where I've found myself with my eyes locked on someone's chest and it's really not the intent.

[JohnBooty]

  It is unfortunate that when talking to females, this 
  puts your eyes in the region of their chest, and there
  have been instances where I've found myself with my eyes
  locked on someone's chest and it's really not the intent.

I get it. This was totally me in high school. Probably in college and beyond a bit, too.

But there are objects in the room besides her eyes and her breasts. Make eye contact, look at something else, eye contact, etc.

Honestly, if all else fails, watch how other people do it. Find a guy who has found a happy medium between "intense and unbroken constant eye contact" and "staring at breasts" and just copy that. I'm not even being sarcastic.

For many engineers this stuff doesn't always come naturally but on the flip side it's all pretty learnable.