There is actually a doll for exactly this purpose:
It appears to be under development by SoftEther Corporation.
However, I imagine such a solution might be overkill.
An alternative would be to use a GI Joe for your modeling. Record video of your GI Joe manipulations from the front, side and top. Using these synced videos, you can go frame by frame switching between a front, side and top view of your 3D model (or you may be able to do it with two cameras). For each frame align your model to match the GI Joe in the recorded video for that frame. This is similar to rotoscoping for 2D, but you're doing it on each axis to expand it to 3D.
Another alternative would be to use a 3D camera like the Kinect.
Remember that you can always elaborate on the animations when you're creating them. You don't need to do a backflip in front of your Kinect to have your character do one. The accuracy may not be great, but it'll give you a good baseline to work off. (However, if you do attempt the backflip, you'll have some sweet 3D video of it.)