I use the magents from K&J Magentics and they are great! They are super strong and with all of the plastic weapons you have no problem with slippage. The only thing I saw was with metal weapons (ABFP and CIB), they can slip on occasion due to weight.
I use a Dremel drill with a thin grinding bit to carve out the whole on the out side of the arm on regular crusis suits. Keep check the magnet for fit and try to get it more or less flush with the surface of the arm if not slightly raised. The round magents work pretty well for this. Using either the D31 or D301's
On forge world crisis suits, it is the same for the outside of the arm, but you can also put magnets on the end of arm where the formworld weapons attach normally. For this I use the B4201's. They are pretty thin and retangular and fit perfect in the space at the end of the arm.
The B4201's can also be used on the shoulders of any suits. I fill in the slot with green stuff, then on of the flat magnets on each should as the mount. It raises it up about 1/16th of an inch so it works pretty good.
For the weapons, the plastic ones are pretty easy to mount the magents on. I use the rectangular B4201's again here since most of my mounts use the same magnet. I usually mount the magnet on the bottom of the weapon so it can be used as a shoulder mount or arm mount. For missle pods, I carve of the small mount and make the bottom flat, then super glue the magnet to the bottom. When mounted on the shoulder magnets, this will set your missle pods 1/8th inch off the modle, which is a good height.
I will post some picture soon hopefully of my suits with the magnets in place.