Okay all you people wondering how to do this technique, I'm finally getting around to posting this tutorial. I've tried to be as thorough as possible. Let me know if you have any questions.
I usually do this at the arm seams and sometimes at the center back when there's a zipper and I can't do a french seam. This first picture is of the armhole seam of the current top I'm working on. I'm starting with a 5/8" seam allowance.
You want to trim the seam down to 3/8" to 1/2". I never measure, just eyeball it. I do trim one layer of fabric at a time. This is especially important around an eased in sleeve, where one layer is gathered and the other not.
Okay, both layers are now the same width.
Looking at a piece of bias tape, observe that one side of the fold is always just a tad narrower the the other. You want to sew on the narrower side, which insures the wider underneath side will be caught by the stitch as well. First you need to cut a piece of the tape that will cover the seam. Don't stretch the fabric or the tape when you are doing this. And cut it a bit longer then you think you need - maybe an inch or so.
Now fold the end under about 3/8" and iron.
Then you need to trim the sides of the folded piece away.
Now pin. Remember, the narrower side of the tape is the side you pin and sew on.
In this picture you can see the side you folded under where you will start the machine. I like to leave the other end unpinned for right now.
Here's what it looks like so far:
Now cut your other end the length you need to meet the first end, leaving another 3/8" to turn under.
Cut the sides of this piece at an angle like you did the other end.
Now stitch and try to get your two pieces to meet. This is close enough, especially if working with a knit.
Slipstitch the ends together, and this is the end result: