First off, let me show you how I hold my needles. I don't honestly know if this is a factor; all I can tell you is that holding my handful of dpns UNDERHAND feels very comfortable and helps me get keep everything positioned snugly at the joins.
Second, I knit with the RH working needle positioned ATOP the preceding needle (the one on the right of the photo). See? In this next photo, the RH working needle (the horizontal needle) is receiving stitches from the far left needle. By positioning the RH working needle this way, the yarn travels the shortest possible distance from one needle to the next. If the RH working needle is held UNDER the preceding needle, the yarn has a tiny bit farther to go to get to the first stitch on the next needle--just enough to make a visible ladder over time.
Third, I use my right forefinger to lock the RH working needle into place, right up against the preceding needle, while I work the first three stitches after the transition.
No ladders, no ridges (the opposite of ladders, when the join is too tight), just smooth, even stocking stitch.
FYI, the yarn is from Opal's "Little Prince" series. As usual, the computer cannot do justice to the lovely antique feel of the slightly faded colours.