Before I stitched up the casing at the top of the dress, I cut a piece of embroidery floss about 5" longer than the casing length. I tied a safety pin tied to one side and put the embroidery floss in the crease at the top of the casing. Then I sewed it shut, pinned the strap to the embroidery floss and pulled it right through in a matter of seconds. Saves so much time an frustration! Here's some photos of the steps:
1. Place piece of embroidery thread in the crease of the casing with a tail sticking out on both sides.
2. Fold over top and then stitch down.
3. Tie the safety pin to the embroidery floss, then pin it to the strap. Then pull it right through!
You can use the same concept for making straps or tubes that are sewn right sides together then turned right side out. Just cut the fabric for your tube about an inch longer than you'll need. Before you sew it shut, put the thread inside the tube running the whole length with a little extra sticking out on both sides. Stitch one end of the tube shut so the thread is attached. When you're done sewing just pull the thread and your tube is now right side out! Then just cut off that end off of your tube.