I actually magnetize every (fantasy) model I own. I used the website Hutobega just linked and got 500 ZD1 6mm x 1.5mm N35 Grade magnets (check the surplus section) for $20. They come to 5cents each...can get smaller amounts but they are ultra useful.
They fit perfectly into the base of 20mm and up fantasy bases, including characters' slotta bases on one side or the other.
As for the magnetic surface to use I tried both sheet steel, and steel paper. And let me tell you, steel paper rocks. It's a tad pricey at about $11 for 2 8x11 sheets (you can actually get quite a few trays done with that much tbh). However, it is ultra thin, simple to cut as a good pair of scissors should work easy (I used the tin snips I was working sheet steel with so it was nothing). While I haven't tried painting the steel paper yet, I'm pretty sure it will hold paint much better than a sheet of smooth metal as it is a rougher, matte surface.
You can find Steel Paper here: http://www.renaissanceink.net/catalog/product_info.php?cPath=1&products_id=10 for $10.95 per 2 8x11. I'm not sure if there is cheaper stuff to be had since it's very difficult to find this stuff as it is.
With this set up you can easily flip your trays upside down (just don't drop them like I did heh). Put them on terrain at any angle, etc.
HOWEVER, metal models are too heavy for this to work on them and will slide off at 10-20 degrees fairly easily. This could be one reason to use sheet steel over steel paper as the bond is far stronger (the models have a noticeable "snap" down to sheet steel, where is they kind of are just "attracted" to steel paper) and will allow you to do metal models inverted and they will stay.
Keep in mind, however, that using plastic dudes on sheet steel will likely see them getting damaged from you "pulling" them from their trays. Since the bond is stronger, it follows that removing the models requires more force and plastic models will likely eventually come apart with chronic pulling at the weakened bonds (skeleton necks and shoulders for example).
I would say to use steel paper for plastic guys, even with metal characters in there, and if you do have a full metal regiment feel free to use a bit of sheet steel instead as it's much more effective on the heavier models.
Have fun!