I'm one of those who hate doing STD. It makes me tense up. Once I realized I could use a serpentine stitch instead, or just go "organic" and do wavy lines with my walking foot, I quit STD completely.
My first piece of advice with STD is to go slowly. When I started STD, I thought it could go fast (and maybe it can for other people). In order to stay in the ditch, however, I found that I had to go really slowly and stop often (with needle down) to re-arrange my hands and the quilt.
My second piece of advice is to resist pulling the seam apart and resist trying to manipulate the quilt sandwich too much. You don't realize it until afterwards, but too much pulling and pushing while sewing causes distortion in your quilting lines. If you do too much manipulation, it can even cause your stitching lines to ripple the quilt.
I found that using a walking foot helped me reduce the tendency to over-manipulate the sandwich. Guiding the quilt is good; man-handling the quilt is not!