How can I restore the smooth surface of my iPhone?
Any surface preparation you apply will not stay on there long, and may well be just as annoying as it rubs off on your clothing etc.  Most of the lubricants I have seen used (often for ski/snowboard prep etc) are nasty horrible things that are designed to impregnate the surface to allow for low friction.  There is no way you can ever impregnate the glass, so this techniques are doomed to failure.  Also, I would be careful with what you apply too, as you may damage the oleophobic coating that is on the glass as standard.

I would recommend simply washing the glass with a very mild detergent, and then wiping clean with a lint free cloth.  If your glass remains rough to the touch after this, then it is likely a permanent feature caused by excessive wear, and cannot be rectified.