If you use anything that can have different load-outs in your 40K army (Carnifexes, Crisis Suits, IG Tanks, etc) you should assemble them using magnets in the locations that you can change. My Carnifexes have magnets in each shoulder, neck and back. The questions I always get is where did you get those magnets? Well, I found a great source for a cheap price and I thought I would share. Check out thinkgeek.com. I got this assortment of 87 magnets for $15. Not bad at all.
Now pay attention kids, I would recommend not putting magnets in both ends of your joint. Reason being is that it uses up your precious magnets twice as fast and if your polarities are opposite on one of your options, it's no longer an option! So I bought a box of 1/4" steel Cut Tacks from Lowes. They look like tiny tiny nails. Snip off the nail part and you are left with the flat head. Glue that where you want it to touch the magnet and (Click!), you got a functional option.
Hope this helps all those of you that have those models that you want to maintain flexibility on (especially since GW likes to redo codexes and the things that you didn't use are now the best option).