Barbara Walker's Knitting from the Top is a fantastic resource that I highly recommend. On page 20 she outlines ten ways to make double increases, my favorite being #2. It is a symmetrical stitch, making one stitch into three with a small eyelet at its base. She describes it as (k1, yo, k1) in the seam stitch. I call it a sdinc in my patterns, or symmetrical double increase.
Here is a series of photos showing how it is done, as if one were knitting Round 31 of my Eileen Mary sweater. It all happens between the markers. Click on any photo to see larger, and do admire my dyer's manicure.
1. insert right needle in next stitch as if to knit,
wrap yarn as usual
and bring loop to front, but do not slip new stitch off of left needle. (k1)
2. bring yarn from right to left between needle tips,
bring loop to back as if purling,
but do not slip new stitch off of left needle. (yo)
3. wrap yarn back around as if to knit
and bring loop to front (k1),
and slip the whole three stitch cluster off the left needle.
Here is the final product. If this technique were used without all the other decorative stitches, the result would be a series of stacked small eyelets, a "handsome increase" in Barbara Walker's words, indeed.