To prevent seams from coming undone, add a few backstitches. Here's how.

Start your seam as usual, taking four or five stitches ahead. STOP. Press a button or move a lever or other control to reverse stitching direction (refer to your machine's instruction manual for details), take a few stitches back on top of the stitches you just made--three or four is plenty. STOP. Proceed with forward stitching. These extra stitches offer assurance that the seam will not come out. Use backstitching at both the beginning and the end of a seam, and when you are not sewing to the edge of the fabric, as with inset seams.