I had this problem too and still do sometimes.  If you pop and your shoulders are rotating ahead of your feet, you shouldn't hit your tips on the rail.  If you are hitting your tips it means that you aren't setting the spin enough with your shoulders and you are trying to create rotation by whipping your feet around.  That may not make sense, but it is pretty hard to describe.  A friend told me to turn with my shoulders and look back up the hill at the top of the rail, then pop and spin.  The first time I did this it worked.