Do you have a walking foot? You can make wavy, curvy lines with a walking foot and not have to worry about accuracy or stitch length or how good your FMQ is.
Are you using the same thread in the top and bobbin? That helps a lot so that a tension problem in the stitch doesn't show so much. Also, some machines won't do a good job with an embroidery stitch used on a quilt sandwich.
If this is a very large quilt, you can peel back the top and backing from the side and cut the batting into thirds. (Use a curving cut and be sure not to catch the backing or top in the cut.) Use registration marks or pin a note to the top of the batting so later you can re-assemble the batting exactly as it was. Do this on each side, and you can quilt the middle to within 6 inches or so of each side. Then re-attach the batting on one side (with a tacking stitch by hand, or with a wide zigzag by machine), smooth the top and backing back in place, and machine quilt that side. Marti Mitchell has a book out on how to do this, but this is basically all there is to it. This greatly reduces the bulk under the arm of the machine while you are working on the middle of the quilt.
To sum up, you can make this task a lot easier by: (1) using the same thread in top and bobbin, (2) using a walking foot, (3) switching to a regular straight stitch, (3) making wavy, curving quilting lines rather than straight ones, and (4) working on only one-third of the quilt sandwich at a time.
HTH!