If you mean the front crease that comes in some pants, chances are good that you won't ever be able to get it completely out. The reason is that in manufacturing or ironing these, the fibres on the crease line are actually broken and bent. So no matter what you do, there will always be a tendency for the fabric to bend there. However, the best way I've found to get creases out of cotton is to wash the item in delicate or by hand so that the spin cycles don't impart creases to the clothing, then let it drip dry or tumble to the point of being damp. Then get your iron as hot as the fabric will take and iron the wrong side of the fabric (just in case of fading etc.). The important thing is to use a dry iron and to iron until the garment is dry, and then once each flat section has been ironed out, allow it to cool without moving it at all. To put it another way, iron a spot until dry, leave it stuck to the board, let it cool off, move the fabric and repeat until you have ironed everything.