I generally avoid flex agent where possible as I prefer to colour sand and
polish the paint when done (self-proclaimed perfectionist).  I find the
flex agent paint doesn't polish well.

My favourite these days is to use 3M urethane paint protection film that is
applied to the finish afterwards.  It does an extremely good job of
protecting against impact ... and can be easily removed.  Of course, it's
rather expensive, but a little more tolerable if you buy a roll.  We have
salt/sand/gravel to deal with in the winter, and the 3M film holds up
better than anything I've used before.

For those out there with a "bra" on the front of their vans, this is a nice
alternative that doesn't scuff/scratch your paint, doesn't have to be
removed for cleaning, and looks good too (transparent). Here's the 3M blurb:

http://www.3m.com/us/about3M/innovation/scotchcal/index.jhtml