Machine sewing a garment or project is quick, strong, and secure in comparison to hand sewing, but the first and last stitches of a machine-sewn seam are the weakest. And as a result, most sewers do something to prevent these more vulnerable stitches from pulling out.
If your sewing machine can stitch in reverse, you've probably grown accustomed to reaching for that reverse button or lever at the beginning and end of most seams. It sure is fast and convenient. But depending on how you use that reverse capability of your machine, it's not always the best choice - and it's certainly not the only alternative to securing the first and last stitches of a seam.
In order to choose the best method of securing stitches, it helps to consider the following issues:
Reducing bulk is always a concern for sewers. Even though we don't think of thread as being bulky in and of itself, when several lines of stitching are on top of each other, the result is a lump that can be felt. Multiple layers of thread are of special concern when sewing on very lightweight or sheer fabric, simply because the bulk is even more apparent. And intersecting seams pose another kind of problem: because the seam allowance of the second seam is frequently trimmed out, only a few stitches will be securing the top of the initial seam. (For example, picture where the waistband crosses the side seams of a skirt.)
Backstitching. For many sewers, the easiest way to secure the beginning and end of seams is to backstitch. Instead of starting to stitch at the cut edge of the seam, sewing several stitches, reversing, then sewing forward again (which results in 3 layers of thread), try placing the presser foot 3/4" from the cut edge of the seam, begin sewing in reverse to the cut edge, then sew forward down the length of the seam (2 layers of thread). When you reach the end of the seam, backstitch 3/4" and remove the work from the machine (without stitching forward again to the cut edge). Since the thread ends aren't at the cut edge of the work but are slightly toward the interior, it may take an extra second or two to cut them - but the advantage is only having two lines of thread on top of each other. And these two lines of stitches will do the trick - there's no need to go back and forth multiple times.
Using a very small stitch length. Since short stitches don't pull out as easily as longer ones, reduce the stitch length at the beginning and end of the seam. How small should these stitches be? It will depend on the type and weight of the fabric, and how much stress will be on the seam. A normal stitch length is 10 to 12 stitches per inch, so 15 or 16 stitches per inch is often sufficient. To be sure, make a quick test sample on a scrap of the fabric in question - it will only take a few seconds. Besides eliminating multiple lines of stitching, this method is an excellent way to prevent the initial seam of intersecting seams from pulling out.
Stitching in place. Many newer models of sewing machines have a setting that allows the needle to make several stitches in the same place. Whereas this causes a slight bulge where the stitches are taken, it does not add the bulk that multiple lines of stitching create. This method is not good for the first seam of intersecting seams. And be careful with lightweight and sheer fabrics, which tend to get pulled down into the throat plate - but consider using it on medium to heavy fabrics in the right circumstances.