These days it seems like formal dresses for sale are designed to bare as much skin as possible. For girls who want to dress more modestly, whether it be for religious or personal reasons, a bit of creativity is necessary. Here are some things you can do to make a strapless gown more modest.
Add straps to this dress. That alone serves to keep it in place. I also added gold gems to the straps do dress it up. If your goal is to just keep your dress firmly in place that is all you have to do.
To cover up a bit more skin, add a shawl by folding it in four lengthwise. Finding the center point, pin it to the center front of the dress.
Create a shawl collar by draping the shawl over the shoulders and pinning it in place. This works best if you wear the dress and have someone help you with the pinning. (ou can pin it as is, or use and iron to make pleats in it.
When the dress shawl is pinned in place, take the dress off and sew it. You can have short streamers going down the back of the dress, cut them off, or tuck them in if you want to remove the shawl later.
Purchase or make a simple bolero shrug that coordinates with your dress. You can use hood and eye clasps or pretty jeweled pins to keep them in place.
If your sewing skills are fair or better, you can make a simple cape addition to your dress with or without sleeves. Add lace trim to the cape and the dress bottom to pull the look together.
Even someone who sews poorly can add cap sleeves to a strapless dress. Make a triangle that measures as follows. The long side measures over the shoulder from the from the front of the bodice to the back. Measure across three to four inches at a right angle. The third side will join the two ends. Attach this to the dress with the long point in the front.
For maximum coverage without ruining the integrity of the dress, hire a tailor to sew an upper bodice and sleeves for your formal dress as shown in this illustration.