Last time I bought trousers (about two years ago I think) I tried one technique which consisted of sewing an elastic band to the waist band of the trousers. It worked well enough for me to wear them (with additional belt) during those two years. But I wasn't entirely satisfied, it didn't look nice and had been a hassle to sew.
Yesterday I went shopping again because my previous trousers were too worn (I am planning on upcycling them into short trousers for summer though!). The "gap" still being there, I looked for other ideas on how to get rid of it. There is another technique where one makes two holes to slip through an elastic band and this band is sewn with the machine on both sides to the trousers waist. It seems like a good idea, but I do not like that you cannot adjust it at will. Since I've had my two children I have the tendency to gain and lose weight every few months *annoyed sigh*.
|A pair of scissors, needle and thread, two buttons, a safety-pin and elastic-band with button-holes|
|Cut a hole inside the trousers waistband|
|Sew your two buttons a little further away from the hole|
|Button one end of the elastic, attach the safety pin to the other end, and push it through the hole|
|Tadaaaa! You're good to go after an under 10minutes job!|