Slouching, Hunching and the Pains of Being a Tall Lead

Being significantly past 6 feet tall, I thought I would have a disadvantage when dancing salsa. When I started I was (and still am) consumed by concern for my follow's comfort. In my efforts to reduce her exposure to my awkward gangly body, I began to bend at the waist and neck whenever a pattern would require me to move under our arms. This is one of the first moments when I realized that salsa is more than just a dance, but it can teach you lessons about life in general in the form of metaphors. This was in fact, one of those instances. Just like in salsa, I had gone out of my way far too often with women (in various ways in my relationships) to make them "happy" when actually I was making things worse. Back to the topic at hand; I realized that once I focused on my posture and disregarding my height as a hindrance, I found that patterns were suddenly executed, smoother and effortlessly. Below are some tips:

1. It would take a midget for you to not be able to walk under your follows arms. Think about it; your ability to walk under her arms is a factor of your height relative to the height of her outstretched arms, plus her heels and a few miscellaneous factors that I'll get into like hand angles. Unless she's 4'6" and you're 6'6" it's highly unlikely that you can't just ignore that height difference for most patterns, because very few follows fail to meet this criteria. Get comfortable, and just walk your arms, as if she wasn't even there - you'll see it works!
2. Hand angles are that much more important. Any time a guy walks under arms, or through a tunnel, at one point they'll have to execute the "waiter" hand position. Shorter guys can get away with not using proper technique, but taller guys can't. You'd be amazed how a waiter hand position and a finger tip lead, will make things better.
3. Use the angles your elbows can make to get through some patterns. Bending at the elbows will help a lot for moves where your back is faced to her and you're trying to make her turn. This will significantly make it smoother and also relieve the lower back pain you're probably feeling at this point.
4. Stay close! Imagine a two handed turn for the gentlemen, with an axis drawn through the hands, down through the body when it's walking under them and to the feet and the floor. If you're center is off of that axis, then your posture will have to adjust to make up the difference. You could be pulled off because of poor following on her part, but most likely, it's because you've stepped too big in the first place. So I recommend constantly making an effort to stay close and step small.
5. Don't lead her so far away! Just like the above, if you're leading her further away from you, it'll be tougher. You have to focus far more on distance control than smaller leaders, even though you can make up for it with longer arms and slouching. Stop it! Getting better at distance control is paramount to any tall leader and it should be your number one priority in my opinion, since so many of our issues stem from stepping too big, and getting too far from our follows.
6. Choose taller follows. If you know you have a psychological barrier to leading smaller women, then don't dance with smaller women. Dance with the taller ones and get into the habit of not slouching, until it is ingrained in muscle memory. I personally would rather you tackle the psychological barrier head on, but whatever works...
7. The most important thing to understand is that you can't constantly sacrifice yourself for her sake. While basic closed position is an exercise in sacrifice, not everything has to follow that example. If you're more comfortable and are able to move more freely, she'll feel and sense that smoothness and gain confidence in your lead. 

So some people recommend bending your knees, and while I have my moments where I THINK I have no other choice, rarely is it necessary. At 6'3" I'm finding that I rarely have to bend my knees at any point to adjust for height. I think of it as a last resort. 

Height is all psychological. You go through your life looking down on people and you just get used to the world being a smaller box you have to fit into, but the second you stop caring, throw your chest out, and just put your hands and arms where you want them it really becomes smooth.