This cast on leaves a nice, stretchy edge. It is specific to k1, p1 ribbing.
Tubular Cast On With Waste Yarn

Getting Started
Step 1
Begin with a length of waste yarn. Cast on half of the total number of stitches you need.
(If you need an odd number of stitches, cast on half + 1 stitch.)
Cut the waste yarn, leaving a 6 inch tail.

Step 2
Change to the working yarn and complete the first row as follows:
*Knit 1, yarn over.
Repeat from * to last stitch. Knit 1.

Step 3
Complete the second row as follows:
*yarn forward, slip one stitch purlwise, yarn back, knit 1.
Repeat from * and slip the final stitch.

Step 4
Complete the third row as follows:
*Knit 1, yarn forward, slip one stitch purlwise, yarn back.
Repeat from * and knit the final stitch.

Step 5
Repeat the second and third rows.

Step 6
Change to K1, P1 ribbing. On your first row you will begin with a purl stitch.
Once you have worked on several more rows of ribbing, you can carefully snip the waste yarn and extract it from the cast on edge.

Step 7
You will be left with a stretchy edge that is well suited to your ribbing.
