I like to look up when I'm taking photos. I've had several posts on this before--you just never know what you'll see or the different perspective you'll have than if you were far away.
One thing I have learned over time is to fill the frame. From the beginning, I was not afraid to fill up the entire frame I was looking through. I don't know why this came natural to me, but it did. Ever since I've been trying to refine this skill.
Three short tips I would give:
1. When you think you are close, go closer. Some of my best shots were getting up a lot closer than I felt comfortable doing. With the shot above, I ended up using my 50mm 1.4. Now, I if I had pulled out my 85mm 1.8--I wonder what I would have caught! I end up rarely using my 85mm because it makes me feel uncomfortable and awkward, but when I do I'm almost always surprised. Especially in portraits, the best photos are getting close to your subject. Don't be afraid - just do it!
2. Take time to look at everything in your viewfinder. Yes, with the digital age we have all become snap-happy with no consequence (i.e. running out of space), but it is good to look at every angle inside of the viewfinder before you take the picture. I even find myself looking into the viewfinder and moving around (which is probably not that safe), but it allows me to see things differently, from different angles before I take the shot.
3. Think lines and symmetry. I LOVE symmetry, especially in photos. Yes, sometimes you should break the rules, but symmetry can add so much to a photo. The photo above for example, is not perfectly symmetrical (because I shot it off-centered), but the lines are symmetrical. The lines of the subject are beautiful and all lead out and in at the same time.
When you think about a portrait, think about the lines of your subject--how you position them matters. The angle you stand matters. It all matters when composing your shot.
What you don't need to do now is to analyze every shot (because life will become boring :), but it is good to think about these things, especially when you are leisurely taking photos and you can take a moment to think about what you're doing. And eventually, over time, it'll become easier and you'll naturally do it!
So now, go pick up your camera, fill up the frame and have fun!