They make a magnetic primer that just paints on the wall under your paint.  I have not personally used it but in theory its by far the easiest option.

![enter image description here][1]

http://www.walmart.com/ip/17771075?wmlspartner=wlpa&adid=22222222227015524551&wl0=&wl1=g&wl2=c&wl3=18152521990&wl4=&wl5=pla&wl6=40014631630&veh=sem


  [1]: https://i.sstatic.net/wuxBK.png