During the blender conference I found myself bringing this up a few times so I thought I'd share it with you.
To get the most out of a rig and to avoid problems down the line, you need to make sure that the model you are rigging is in a suitable default pose. Almost all the models you see being created are in the classic T-pose (for bipeds anyway). Standing upright with legs together and straight and arms out to the side.
I don't like this.
For me the best pose is when all joints are at their midpoints. When ever I'm making a rig I always make sure that it works for poses more extreme than ever will be animated. Cause if it works in a crazy, over extended way, it'll work normally. So having the joints at their mid pose means at the other ends of the pose, the deformation is minimal.
When a model is in its T-pose, the arms are at their most extreme pose. A lot of people miss this. The arms shouldn't rotate higher than perpendicular from the body, that extra hight comes from the shoulders. Ask any rigger and they'll all say that shoulders are the hardest things to get right. I've found that using this method solves most (definitely not all) problems.
I'm using the shoulders as an example of the rule. You should keep this in mind when rigging any part of the character, arms, legs, fingers... Feet are probably the exception to the rule as its easier to animate if the feet are flat, because they will be most of the time. Also keep it in mind when rigging different types of characters. Bipeds, quadrupeds, an octopus, a giant alien with two bodies, one neck, three feet and an eye on a tentacle....
This is just my opinion and when it comes down to it I'm usually not that picky. If I'm given a T-posed model to rig, ill generally just do it. But preferably id like it all mid posed. Just something to keep in mind when you start your rigs.