OK, so the down-and-dirty elastic trick doesn't suit you for helping you fit your tiny waistline. It's on to Part 2!
It helps to have a friend to help you analyze the front and the back fit of your skirt or pants. As I said before, some women are quite tiny right at the spine in the back, but sometimes waistlines can be reduced at a variety of places. Using straight pins, have a friend experiment with you on different areas to reduce.
My number one rule is to AVOID THE ZIPPER. Even after 47 years of this, zippers are not my favorite part of sewing. If the zipper is at the side, I usually reduce the garment at the center back, or in the back where darts would be (about 4" on either side of center back.) If the zipper is at center front it's easy to reduce the waist at the side seams or anywhere across the back, so that is the best case scenario.
The one time you will probably HAVE to move the zipper is when there is a side zipper, and the hip curve of the garment needs to be changed. Then you will have to remove the side zipper, re-fit the side seam (and the waist) and then re-apply the zipper.
For now we are going to assume that you can avoid the zipper issue.
Step 1: Working with a fitting partner, determine by pinning where the best place to reduce the waistline might be. For now, let's assume it is at the center back seam.
Step 2: Using a large safety pin or large T-pin, have your partner pin the waist until it is tight enough to suit you. Pin through the total waistband (that's why you need the big pin!)
Step 3: Have your partner pin taper the center back seam towards a suitable fit at the hip line. This can also be done on both sides of true center back (about 4" out from CB,) just as long as the result is pleasing.
Step 4: Remove the pants and working on the inside of the garment, chalk a mark across the pinned fold in the waistband. Also chalk mark the pinned tapers, wherever they are.
Step 5: Remove the pins.
Step 6: Mark the center back point of the waistband if that is where you are going to take it in. If you chose to adjust the waistband at the side seam, mark that point instead.
Step 7: Unstitch the waistband from the garment, going about 5" out on either side of your mark. It is not usually necessary to remove the whole waistband, although you can if you prefer. I'm always looking for shortcuts, so I normally work at the rear or sides of a garment, and I do not remove the whole band. Also at this time, remove at least the upper edges of any belt loops from the waistband. You may have to completely reposition the belt loops if your tapers interfere with their positions, but at the very least you will need to release them from the waistband, leaving them sewn to the garment.
Step 8: Measure the distance between the two chalk marks that are now showing on the waist band. Let's say it's 3". Divide that distance in half (1.5") and make a note of your number.
Step 9: Cut through the whole waistband, front and back, at the point you marked in Step 6. There may be top stitching on the upper edge that has to be removed, the waistband may have a back facing, but in all cases, you want to deconstruct the waistband about 4-5" on either side of your cut.
Step 10: The order of work here is important. Next, reduce the body of the garment, following the pin marks that indicated the taper down from the waistband. If you're just working at the center back, this will be a single seam. If you're working at the dart positions on the right and left of the center back, there will be two seams. Refer to the measurement you got in Step 8, and make certain that the upper end measurement or measurementS of your tapers equal the full amount of the waistband reduction. In our example, your total amount of fabric reduction at the top edge of the tapers must equal 3", which is the amount which you are reducing the waistband. Remember--reduce the body of the garment first.
Step 11: If you wish to remove the seam bulk from the seams you just sewed, do so at this point.
NOTE: In my experience, a woman's life is one of being "an accordion." We go in, we go out. We gain, we loose. If your body fluctuates, and the garment in question is a classic, expensive garment, I usually opt to retain as much of the seam allowance as possible. This way later, the seam can be let out if necessary.
Step 12: Next, reduce the waistband by the amount determined in Step 8. For our example, that was 3". We divide that 3" by two, which equals 1.5". Pin the cut edges of the waistband right sides together and sew a seam equal to one half of the total reduction. In our example, the seam width will be 1.5".
NOTE: Depending on the type of waistband on your garment, you may have to do Step 12 twice, once on the inside facing of the waistband and once on the outside piece. If it is a fold-over waist band, you will just make one single seam.
STEP 13: Reduce the bulk of this seam if you must. Again, I try to leave room for later adjustments.
STEP 14: Re-apply the waist band to the garment, matching the manufacturer's sewing and top-stitching processes. Re-apply belt loops if necessary.
Now, enjoy your lovely, tiny waistline!