Making good photographs requires learning a wide variety of skills, some technical, some artistic, some social. For me, the absolute "hardest thing" to master is posing.
I've often felt exactly like US Supreme court justice Potter Stewart must have felt when asked to describe "obscene" materials -- he couldn't but famously quipped "I know it when I see it." When a pose works, you know it. But getting from the subject standing in front of your camera, ready to be directed, to that "know it when you see it" pose is the real challenge. I'm far from being an expert, but thought I'd share some things I've learned about posing.
First, and this may be controversial, avoid any of the hundreds of books available with titles like "Posing Guide." I've either purchased or borrowed at least a dozen of these, and out of all of those found perhaps two or three really useful ideas. For the most part, I find them to be guides to mediocrity and sameness, which might better be titled "How to make everything you shoot look like Olan Mills." If that's the kind of photography you want to do, great. Not me.
My main advice: Steal. Blatantly and often. Not photographs, of course -- poses. At least as starting points! Spend some time looking at the work of photographers you admire or just happen across, and when you see a photo where the pose works, save a copy of it to a "poses" folder on your hard drive. You could even divide the folder up (like I did) into individuals, couples, weddings, groups, families, etc. Then next time you have a session coming up, take a quick look at it, jot down a few ideas from it on a notepad, and use that as a starting point for your own session. Don't copy what the other photographer did exactly, analyze it and figure out WHY it works, and then apply that same reasoning to your own shots, and your own subjects. To try this out, just head to google, search in images, and enter "photography couples." You'll get a great body of material to refer to, as well as some that obviously don't "work" at all.
My personal favorite photographer to use as inspiration is Herb Ritts. Herb (recently deceased) was a celebrity photographer, but the appeal of his photos doesn't come from the celebrities in them, it comes from the way he poses them and how he gets very specific emotions out of them. It's extremely rare for me to see one of Herb's photos and think, "Oh, sure, that's just like something I've seen before." Of course, exactly copying Herb's work isn't what I'm after -- his genius was often simply not being afraid to try something nobody had ever done before. His work inspires me to do the same, but just perhaps get some starting point ideas from what *he* did before.
Once you've begun to find your own kinds of poses that work, be careful about over-using one or two "good" poses. If your entire portfolio starts to look like the same two shots, just with different people and backgrounds, people will notice (including you!). I've run across this myself -- a particular spot in my yard with a gorgeous old oak tree is a wonderful location, and the tree is just the right height for people to lean on it with their arms (see this photo from my post on outdoor lighting). After I first did this pose with one of my senior portrait clients, and it *worked,* I started using it for others. Too much. About halfway through the season, reviewing photos, I realized I had no less than 10 kids posed in that same spot in the same way...and that it was time to try something different. That same idea is the reason I suggested avoiding all of the posing books that are available, because what most of them teach is "this is a posing rule, this works, use it" -- and then your pictures look like everybody else's that's read the book.
So keep refreshing your sources of inspiration, keep trying something new, and don't be afraid to really go way out there on a limb. If it doesn't work, no harm done. If it does, you'll almost certainly know it when you see it.