Destri from The Mother Huddle shows how to make an easy knit waistband with elastic running through it. The elastic gives it strength (keeps it from stretching out as you wear the garment) and helps it keep its shape. Get the tutorial from The Mother Huddle.
[photo from The Mother Huddle]
Cheryl says
I save the bottom bands from sweatshirts that I have made into jackets and other things.. also the cuffs.
I use those to make waistbands..and use all the ribbing for other projects.