These are questions I struggle with too.
For me, I like to plan the shot that I want and try to grab it up front. So, for a portrait, I'll start a little wide and work my way in to close with varying expressions. This is the first 20 minutes or so. With some technique- not photographic, but social- I can usually get the subject to relax a bit and most of the time I can land a few safety shots. These are shots that I know the client will accept, but might not be as creative as I want as an artist.
Once I get those shots in the bag, so to speak, then I start getting creative. That's when I start playing with angles, shoot wide open, bring more environment into the shot, work with props. This is the fun part of the shoot for me and the majority of the time the client picks one of these shots as opposed to the safety shots. However, you know you have them, so if you get too creative or they don't come out for some reason you have a back up plan.
1) If you're feeling like your losing your vision or the point of the shot, pull in. Simplify down as much as possible and then take a different route.
2) In order to make sure you're getting the expressions you need, practice "expression cycling". I just made that name up, but a buddy of mine taught me that you take them through a series of expressions rapidly- "Smile- dead serious, smile- halfway- big smile". This helps get people out of that snapshot face thing.
3)Show them what you want. There's no better way to make a shoot fun than to strike the pose you want them to hit yourself. Yes, you'll feel silly but it's worth it.
I look forward to seeing what some other people reply. The shot plan is something I would like to improve on too!