I use either 4' rebar or 4' metal fence posts stuck in the ground. This works if you have dirt (not pavement) and if you are confident you can avoid sprinkler lines. So, rebar and zip ties are great. Will your skellies be dressed? Or otherwise accessorized? That helps hide the supports as well.

There are camo colored zip ties that come in that light tan that blends with skeleton color, or black ones work pretty well, too.

If you are on pavement it is harder because you need to create a stable base. I don't have experience with that. I will have to learn this year BUT my skeletons are wearing monks robes so I can discretely tuck a cinder block at their feet

To pose their arms I have used the wire at Home Depot or Lowes that is used to support insulation between joists. It is like spring steel - very stiff. You can bend it with pliers and a lot of force. And it's pretty cheap (comes in a long, thin box with like 100 PCs). It takes some wrestling but it acts like an external brace to hold an arm in the right position... Using a lot of zip ties again. This is where having nekkid skeletons is easier than costumed ones. You might want to plan how/when you support them based on their attire.

 

Shepard's hooks that are used for hanging pots work well. Cut the hook off and zip tie the rod to the spine. Use 3"screws in the knee joints to keep the legs stiff, you can also use screws in the arm joints if needed.