A Clovis should have a nice center ridge line. Make sure your flakes don't travel past the center of the piece and you'll develop a nice ridge for the flute to travel down. The nipple/ platform should be low and the force of your strike should go in at a fairly shallow angle. A flute is really only a thinning flake taken from the end of the point. Knapper you are correct that supporting the tip should help stop the piece from breaking.