So I was thinking about my Sackboy and how his wire skeleton was made, and I thought I'd write it up in case anyone else wants to wire-frame a creation. This method should work for anything knit or crocheted, and probably would be easily adapted for sewn figures as well.
Start with a single, long enough piece of stiff-ish wire. My personal favorite for this is the spiral wire from a 2" thick notebook. In general, you can figure out how much wire you need like this: the 2* the height of your figure, plus 4* the arm length, plus 4* the leg length, + 2* tail length (if it has a tail).
Begin by bending it loosely in half. Bend the central part of the wire into a circle, then twist the two ends two or three times to form a twisted neck. Then bend the wires outward to form arms - about where you want hands to be, form a loop that will be the hand, then twist the free end around the arm up to the body again. Twist the two free ends together to form the torso; stop about where the hips should be. Bend the wires outward again, and like for the arms, form feet, then twist the free ends around the leg. If you have any leftover, you can use it to form a tail, or just twist the remainder around the torso.
If you have a model that's not quite humanoid, you can easily adapt this - just start from the head and work your way along, doing each limb as you come to it. If possible, using a single long piece of wire gives the best result, because it can't come apart with bending. If you must use two pieces, then try twisting them together, forming the head with the twisted part, and going from there.
See this diagram; sorry, it's hand-drawn, but hopefully it gets the point across. Squiggly places are places where the wire is twisted around itself.
I hope someone else will get some use out of it - and I'd love to see any work done with this method!