I have read of a few gathering techniques used to gather long rows of basting stitches but the one I turn to again and again is one I found in a Simplicity pattern.
The pattern is Simplicity 4140 and the technique is found on page 2 of the guide sheet and entitled "SIMPLY THE BEST GATHERING TECHNIQUE." I'll let you be the judge of that, but this technique has never failed me. Verbatim, the tip reads as follows:
"Loosen the needle thread tension slightly. Use heavy-duty polyester thread in the bobbin. Set the stitch length for a long stitch.
Working on the RIGHT side of the fabric, stitch 5/8" (1.5cm) from the raw edge and stitch again 1/4" (6mm) away, within the seam allowance OR stitch as indicated in the Sewing Directions. Leave long thread tails.
Once the gathered section is pinned to the shorter section, gently pull on the bobbin thread, sliding the fabric along until it fits the shorter section.
Stitch along the seam line."
That's all there is to it. On my most recent projects I have used J&P Coats Dual Duty Plus Extra Strong Thread but I have used other threads. The main thing is that the bobbin thread be extra strong and polyester. I think the polyester helps it slide and also makes it strong.
Half the time I forget to adjust the tension and the method still delivers a flawless line of gathering stitches.
I hope if you haven't settled on a favorite method for gathering you will give this one a try and that you are as pleased with it as I am.
(I will use gathering on my next project at which time I will post a photograph.)