EDITED 9/24/10: The full detailed directions for this alteration are now available on the website here at LearningAlterations.com
Do you know a man who is keeping to his New Year's resolution to lose weight? If his pants are beginning to get too big in the waist you can easily alter them. Most men's dress slacks have a center back seam that can be taken in quickly and leave no signs that it has been altered. The following directions also apply to letting pants out. Just make your line of stitching to the right of the existing seam instead of the left.
First remove the belt loop.
Next, detach the back waistband from the inside of the pants. Below is a before and after picture.
Next calculate how much you want to take in. Divide this in half and mark your new seam line either using chalk, pins or water-soluble marker. For example, if you want to take the pants in 2", mark a stitching line 1" to the left of the existing stitch line. Then stitch where you have marked. Below is an example. I am taking in quite a bit.
After that you have to remove the original seam's stitches. Be careful not to nick the fabric here and be sure to pick out all the little leftover threads. Cut and serge the excess seam allowance if necessary.
Press the pants flat to remove the stitching line.
Finally, you want to fold the waistband back down into place. Then with the right side of the pants facing up, run a line of stitching in the ditch created by the waist/pants seam. You can fudge it a little, as I did here, because the belt loop will be over top of it.
Replace the belt loop and you are finished!