Sometimes even an elastic waistband is too loose. A well-fitting elastic waistband will fit smoothly against your waist when you are standing or sitting and still be loose enough to allow you to slip on the trousers or skirt with ease. Taking a tuck in the side seam will tighten the waist, but it will create an unattractive and uncomfortable lump in the waistband. A better alteration actually makes the elastic in the waist smaller.
Open a 4-inch section of the casing that holds the elastic by using a seam ripper to take out the seam that holds the edge of the casing down. Try to find the point where the ends of the elastic are sewn together by feeling for the end of the elastic. If you can find the end, open the casing at this point.
Pull out a loop of elastic out and pin it together with a safety pin.
Try the garment on and adjust the size of the loop until the waist fits snugly, but the garment is still easy to put on and take off.
Cut off the loop of elastic, leaving 1/4 inch of extra elastic on each end of the elastic band.
Overlap the ends of the elastic 1/2 inch and machine sew them securely together with a wide zigzag stitch. If the elastic is wider than 1/2 inch, sew along both the top and bottom edges of the elastic. Be sure the elastic band is not twisted anywhere along its length.
Fold the casing back into place. Repair the section of the seam you removed earlier by machine-stitching along the original seam line. Stretch the elastic as you sew so that the fabric of the casing is flat.
Distribute the gathers along the waistline evenly.
If the seams around the waistband are sewn through the elastic, you will have to remove all these seams before you tighten the elastic. Fold the casing back over the resized elastic band and sew along the original seam lines, stretching the elastic as you sew.