First of all, if you have a tiny waist, I'm jealous. I've always been built like a tree...first a twig, now a giant redwood. But I have NEVER had a waistline.
But YOU do, and the waistband of your slax, shorts, skirts and jeans are always too big, and it gaps, particularly at the back.
There are going to be several posts on this subject, but we're starting with the quick fix that's the first step up from using that oversized safety pin to close the gap. Our first assumption for this quick fix is that the fabric that makes up your pants or skirt is light to medium weight...i.e. it is NOT denim or heavy wool.
STEP 1: Purchase a length of "no roll" elastic that is longer than the waistband. There are many kinds of elastic, but "no roll" means that it will keep the vertical firmness and not fold over when worn. While there are elastics that are more stretch-y than the "no-roll" kind, they tend to twist inside a waistband and eventually become uncomfortable. Most stores that sell fabric notions will sell elastics by the yard.
STEP 2: Starting very near one of the opening ends of the waistband, but past the point where there is either a hook, a button or a button hole, remove about 2" of the stitching that secures the waistband to the garment.
NOTE: Usually, one side (sometimes the underside) of a waistband is sewn to the garment, and then the opposite side of the waistband (the front of the band) is top stitched in place. Alternately, the front side of the band can be sewn first, and then the waistband is folded over towards the inside, and a line of stitching is made right along the horizontal waistline seam in front . (This is called "stitching in the ditch" and is often virtually invisible on the outside of a garment, but on the inside of the waistband you will see a line of stitching.)
For the purposes of this quick alteration, remove 2" of that second row of stitching that secures the waistband to itself, whether it happens to be on the outside or the inside of the waist band.
STEP 3: Pin a large safety pin to one end of the elastic, and starting at your opening, slowly begin to push it through the center of the waistband area. Push the pin along 1-2", then hold the pin with one hand, and allow the elastic to work its way into the waistband. Continue doing this until you get to the opposite end of the waistband opening. This takes a bit of time, and some coaxing of the elastic.
NOTE: On occassion, belt loops can interfere with this process, particularly if the upper edge of the belt loop is sewn lower than the very top edge of the waistband. To avoid the belt loop issue, a) select a more narrow elastic or b) unstitch the top of each belt loop and re-apply them at the end of this adjustment process.
STEP 4: Just as you did in Step 2, remove about 2" of the second row of stitching at the destination end of the waistband, but stop short of the button hole or button.
STEP 5: Continue to thread your elastic all the way to the point where your second opening is in the waist band seam, and bring the pin out in plain view. Pull hard on the elastic to allow yourself a bit of excess elastic to hold. Remove the safety pin, and slip the raw edge of the elastic up between the two layers that form the waistband. Pin it securely with straight pins, and then, using thread the same color as the garment fabric, stitch through all the layers (2 waistband sections + 1 elastic) two or three times. Do this about 1/4" in from the end of the elastic, and make sure everything is secure.
STEP 6: The elastic is now firmly anchored at one end of the waistband opening, and the 'tail' of the elastic should be hanging out the other end. Pull on the tail to tighten the elastic inside the waistband, guessing about how tight you will need it to be to fit your waist. To secure the second end, pin the tail end of the elastic through the waistband, and then try on the garment.
STEP 7: Adjust the elastic to a tightness that is comfortable for you. Focus on your comfort. Re-secure the end of the elastic with a pin through all the layers.
STEP 8: Securely pin the elastic through both layers of the waistband about 2" in from your opening. Cut off the tail of your elastic, and, as in Step 5, secure the end of the waistband elastic up inside the waistband, again sewing through both the elastic and both layers of the band.
STEP 9: Stitch the two opening at either end of your waistband closed. Begin your stitching right on top of the stitching you had not removed, and pulling on the band if necessary to keep it smooth, individually stitch the two openings at either end of the waistband closed, stitching on top of the still-secure stitches at the opposite side.
STEP10: If necessary, work the elastic gathers with your hands to make them even around the whole waistband. The more spread out that any gathers are, the less they will show when you wear the pants.
The Quick Fix is now done, and you can expect those pants to stop gapping and to stay up better!