A sharp crease in a pair of trousers is a sign of sophistication in fashion circles. It's often a requirement in the military. Sharpen your creases with a few skilful passes of a hot iron, some conservative use of starch, and an old-fashioned bar of hand soap.
Heat up your iron to a high setting and lay out one leg of the trousers on the ironing board with the crease you want pressed tightly together. Apply a light mist of starch along the crease and graze the iron over the crease, applying greater pressure as the area starts to stiffen a bit.
Pull the bottom side of the crease tight as you move from the fat end of the leg down to the cuff of the trousers. Increase the pressure to sharpen the crease, and spray more mist over the area to harden it up a little more. Go with the natural existing crease if there is one, or you will create what's referred to as railway lines.
Wet the end of a bar of soap so it is a bit greasy and reach inside the leg with the bar held tightly in one hand. Run the greasy end up and down the length of the crease a few times. Press the crease together again with the iron and starch it one last time. Iron the crease dry once more with high-pressure passes. As the residual soap boils and caramelises, it becomes a clear wax and an all-natural adhesive to keep the crease both stiff and sharp.
Repeat the same steps for the other leg, and carefully hang the trousers up until they are ready to wear.
Before you iron, if there are excess wrinkles in the slacks, run them for about 10 to 15 minutes in a dryer. This will also get the trousers warm enough for the starch to work even better on the first application with the iron. Spray some starch along the inside of the seam for extra stiffness and durability.
Always be careful using a hot iron. Use quick passes when possible, and don't lean on it for too long in any one spot when applying pressure to avoid damaging your trousers.