1. Know your camera's ergonomics - When your on a session time passes by very quickly. So especially if you're outdoors, you need to be able to move fast to grab the best light(magic hour). Manual is ALWAYS the best(and a must, if your shooting anywhere into the sun), although AV mode can work for beginners. Learn the exposure triangle(shutter speed, aperture, and ISO), and practice adjusting on the fly in manual. Be familiar with the buttons on your camera...doi*. Besides shutter and stop, I most frequently use the spot focus point, moving it around to create different composition framing.
2. Shoot in RAW - The editing opportunity with RAW, far outweighs the inconvenience.
3. Shot Technicals - Exposure, Histogram, and Focus. Nail these 3 things! The first two take practice in manual. The third is hit and miss due to operator error and AF error.
4. Shoot A LOT of frames - Memory is cheap, use it. In a 2 hour session, I can easily produce 600 images. Pause only to review in camera playback to check exposure/Histogram. I normally at least take 2 shots per pose, to ensure I hit focus.
5. Learn to EDIT - I use LR and CS5. If you do portraits, then the Portraiture plug-in for LR is Amazing(you have no idea until you use it). Setup action in CS5. etc etc.. Creating your editing workflow is the single most time consuming aspect of digital photography. Editing is what WILL separate you from the others, LEARN IT.
If I had to break it down, I would say that, to create great images that are 80% better than most semi-pro photographers.... it is 30% Shot technicals, 50% Editing, and 20% composition. Meaning; if you can shoot in focus and at exposure, Edit well... then you will be better than 80% of the "others". That final 20% is what moves you into the upper echelon, and is rooted in experience.
Tips for creative composition?; Join Flickr or 500px and do some research. Find images that look really well, then ask yourself how they could look any better. Write down your ideas! I will sometimes bring prepared notes I have written about certain poses that I want or angles. Again... when your on a session, and time is of the essence... that stuff can go right out the window. Be PREPARED.