There are three main problems experienced with zips. They frequently become stuck, will not stay up, or the teeth become separated. There are several things you can do, short of replacing the zip. Depending on your surroundings there are many fixes for a broken zip. Provided all of the teeth on the zip are still intact, repairs can typically be made.
Remove the trousers to get the best view of the zip and angle for making fixes. Lay them out on a level table or other flat surface in good light.
Check the zip for obstructions. Frequently the fabric surrounding a trousers zip may bunch or pucker, catching in the sides of the zip pull and preventing it from going up or down. Pull fabric or other obstructions, such as hair or loose threads, from the edges of the trousers zip pull with fingers, tweezers or needle-nose pliers until the pull is able to move. Run it up and down several times to ensure proper operation.
Rub ordinary bar soap onto the teeth, or coils, of the zip to lubricate the slide. This will make it easier to move the pull without causing the zip slide to slip down unintentionally.
Zip won't stay up
Remove the trousers and lay them out on a steady work surface with adequate light. Pull the slide all the way closed and fold the pull of the zip down to lock it in place.
Grip the zip slide at the top end over one side of the zip with needle-nose pliers. Squeeze gently to press the gap at the side of the trousers zip slide closed slightly. Repeat on the other side of the zip. This will cause the slide to grip the teeth more snugly.
Test the trousers zip by pulling it up and down several times. Put the trousers on and pull the zip closed. Fold the pull down to lock the zip in place and pull against the edges of the zip to test it. If the zip still slips, repeat the process to tighten the pull more.
Zips that gape open
Remove the trousers and lay them flat in good light. Pry one side of the zip pull open slightly to release it from the teeth.
Position the pull at the bottom of the zip, with the teeth properly aligned and squeeze the zip pull closed with needle-nose pliers.
Pull the zip closed. Open and close it several times. Close the zip and pull the trousers legs at the sides of the zip to ensure that the zip will not gap. Tighten the pull with the pliers as needed.