Here's an easy tutorial for how to add 'rare earth magnet' joints to your customs. I've tried several methods, and this has provided the most secure and stable technique.

Ingredients Needed:
· apoxy glue (I recommend Super Glue plastic fusion)
· dremel kit or a drill
· 1 rare earth magnet (these are at most art/craft stores, or home depot. You can also buy these magnets online at gaussboy's website)
· X-acto knife (or just the blade)
· 1 small washer (the thinner the better)

To begin with, I disassembled movie Starscream's arms. The dremel bit seen in the pic below is perfect for this modification, because of it's composition, shape, and size. I need to get the hole in the should part LARER than the magnet. Sure, sometimes a tight fit seems good, but we want a lot of adhesive between the magnet and the plastic.
I dremel the hole larger using the special bit on my dremel. Afterwards, cut away any shards with my X-acto knife, to give a smoother surface. Loose shard and plastic will cause problems later on.
Next, I take the rare earth magnet, and pick it up with the X-acto knife. (because it's a magnet, it latches right on) I take the magnet, and roll it in my 2-part apoxy glue, making sure to get a heaping generous amount around all the sides. From there, I place the blade over the hole I made, leaving it magnetized onto the blade. This is the cool part (sorta)... Because the blade's edge is flat, it keeps the magnet completely level in the hole. The blade holds it IN the hole and it will never be too deep in or sticking out at all.
About 5 minutes later, I'll rip the blade off the magnet. The apoxy should be dry enough to keep the magnet in the hole. From here, I wait until the apoxy's completely dry and then sand down and excess of glue. On the other end of this joint, I will use a bit of apoxy to glue a standard washer onto the plastic.
That's it, I hope this helps anyone aiming to do something like this. This is a very easy system to create custom joints. Although easy, it's only recommended for light stressed joints. It's also a nice way to make your transformer something where parts can come off easily (commissioner here wanted arms removable in jet mode)