Trampolines are relatively durable, but over time they can lose a bit of their spring. Before you jump to purchase a replacement, consider tightening the trampoline yourself. To do so, you'll essentially need to tighten the cords running around the circumference (or perimeter) of the trampoline. Consider asking a friend or two to help, as you'll need to flip over the trampoline to access its underside.
Open up the vise grip pliers. Then, take masking tape and wrap it around both sides of the pliers' jaws. The jaws should be approximately 1/8 inch apart.
Flip the trampoline over 180 degrees. This may take a few people lifting at one time.
Look inside the nearest knot block and you'll see a bounce cord. Take the cord by hand and pull it inward -- toward the middle of the circle -- so that the knot is toward the middle instead of toward the outside.
Grip the cord with the vise grip pliers where the cord meets the block. This should keep the cord from slipping.
Loosen the overhead knot -- found toward the end of the bounce cord -- using needle-nose pliers. Tie an overhead knot closer to the knot block. The closer you tie the knot to the block, the tighter the result will be. When you're satisfied with your new stretch, you can remove the vise grip pliers.
Repeat the process by taking out knots and retying knots in new positions around the trampoline. For best results, the space between old and new knots should remain relatively the same.