You can do it at the arm/shoulder joints as suggested, but you will have to take care so the arms don't "droop".
I came up with another method, which while perhaps MORE complicated than just magnetizing them arm sockets, I think looks about 10 times better (I don't care for he way the missile arms look).
Basically, I figured out a way to modify the railgun so that it could also be a heavy missile rifle.
Here is my
WIP post on it.
http://www.dakkadakka.com/dakkaforum/posts/list/550313.page#6019513
These things look VERY cool on the table, and I get tons of compliments on them everywhere I take them.