This seems like a good place to store these little techniques so that I can find them again, rather than on the back of a receipt where they currently reside.
The first is for smoother joining of the first row in circular knitting. To avoid confusion, make sure the last cast on stitch is in the right hand and the first cast on stitch is in the left hand. Cast on one extra stitch, slip it on to the left hand needle, and then knit the first two stitches together (k2tog). This should make a strong join without a gap.
The second technique is to avoid or hide the jog when new colors are introduced in circular knitting (which is actually a spiral.) Join the new yarn and knit one complete round in the new color. At the start of the second round, lift the right side of the stitch below onto the left needle, then knit it together with the next stitch (again k2tog). This pulls the row below up a bit so it's more in alignment with the next row. Looks pretty good.