I have a new bumper cover for my wife's 05 Magnum, that needs to be painted. What is the proper/best way to prep the urethane cover so the paint will stick and not flake off in 6 months.

Most people make the mistake of not putting a flex agent in the primer, flaking off is poor prep work or lack of sanding adhesion properties.
On a new "bare" bumper, completely clean with a degreaser, prep solvent,
either sand or heavy scotch bright, then apply primer, sand again, unless it's self etching type,
there has to be a step here for adhesion, ask your paint supplier, what he is selling you.

the bumper comes already primed with flexible primer from the factory ,if it's OEM bumper,I use "Scuff Stuff" paste and red scotch pad ,rinse it off, apply two wet coats of Plastic Magic adhesion promoter and follow with color and clear. Never had adhesion issues ,I love using this stuff, it works great on raw plastic bumpers too

do a google search on these products,most paint supply stores have it in stock .

whatever you do, invest in some adhesion promoter,