Knitting a flat swatch for circular gauge.... huh?

Most knitters may get a dramatically different gauge depending on whether they knit back and forth, or in the round. If you are going to knit a garment in the round, your swatch should be knit in the round. But this can mean a very annoyingly high number of stitches.
Fear not, knitter. Knit your swatch flat, knitting every row, and carrying your yarn across the back. Sloppy?

Yep. But so very clever. And it can mean the difference between a sweater that sort of fits, and one that really fits. As usual, thanks EZ.
Cast on 4 inches worth of your desired gauge plus at least four more to take into account the icky edge stitches.

Use a circular or double pointed needle and knit the first row. Do not turn. *Push your stitches to the tip of the other end of the needle. Draw the yarn very, very loosely across the back of the work and knit the row again. Repeat from * for about 4 inches.

You will now have a swatch in which the gauge is accurate for knitting in the round in stockinette, because you have indeed knit every round. And you can always rip it out later if the messy back is driving you crazy. In fact, that is what I am going to do right now.