Planning usually plays a fundamental role there. “Usually“ because I bet there are some that can skip this part and reach great results like grandmas that don't need a recipe to cook those fabulous meals :-) .
Of course we need to plan animations too...but, I'll just take one step at a time and discuss planning of poses now. To keep things illustrated as we move along, we'll use the theme balance :-)
Reference
A good start is gathering reference, taking into account the theme you're working on. Search google images, reference sites (like http://www.gettyimages.com/), Eadweard Muybridge's books and also look around you, what do people do to express the feeling you want to transmit? Depending on the amount of time available, also check out movies or youtube videos. And, of course...see what YOU would do. That's a model that will always help you and even if you won't always be able to do what you have in mind, most of the times it helps. :-) Take pictures and test different ideas. 
How would we look for reference if the theme is balance? Dances, slacklining, surfing, circus acts, yoga, ...there are lots of possible keywords to find adequate poses. Sometimes you also find poses that may be of use for a future project.Sketching
After you've got plenty of reference, select a few poses that please you or seem more apropriate for what you're working on. Make a few sketches. You don't need to draw all the details, just a skelleton suffices really. Make sure you have the hips and shoulder rotations in there.
Check it for physical principles...is there balance? Is the weight well distributed? Is there a clear line of action in it? These are the important points, you don't need to be a wonderful drawer to make clear sketches (take a look at mine and you'll know what I mean ;-) )
To draw your sketches or evaluate your poses, it sometimes helps observing the negative space: if there's an empty area between the arm and the body, how much space is there between the legs. Sometimes it helps seing negative if you've looked for too long at something for example.
See if you can make it more interesting...can you push it a bit more? Wouldn't it be nice to add some exaggeration to it? In the end, your character will probably not look exactly like the reference you chose because maybe his body is a lot different or maybe he would look nicer if you push the pose a bit more.
Get feedback! Other people will have a fresher pair of eyes and are likely to help you out a lot with that :-)
Posing
Now let's start posing! You'll follow the reference you've got, but since your character is 3D, you can always chose the best angle of your pose. Take a look at the silhouette, is it readable? Do you have a clear view of what's going on with the legs, arms, body?
If you can understand roughly what's going on only by looking at the silhouette, you're on the right track! Always be sure the camera angle you chose lets you see the line of action - can you draw a nice clean line through the character's body. More about line of action here.
So here's my pose after all these steps:
By the way, the pose I chose comes from one of my favorite videos on youtube, so I thought I'd just share it with you...it's Polina Semionova dancing to Herbert Grönemeyer's Letzter Tag, really beautiful :-)
