Not perfect, but WAY better than it looked before I started (I would have never let my header get this terrible, but this is how it looked when I bought the bike)
800 grit wet sanding. Then 1000 grit wet sanding. Then Metal Polish (Turtle Wax brand) with polishing ball + drill
Then electronics cleaner (any alcohol based, quick drying cleaner) to remove residual oil from touching the pipes.