I hesitate to wear strapless dresses because my I'm a major pear shape - my hips are way larger than my chest, so the tops are usually too big. I spend all day hiking up the top (totally tacky).
However, I dug deep into my inner Little House on the Prairie to come up with an easy solution to the problem that you can do even if you're not much of a seamstress. It only takes a minute!
I've illustrated it above using contrasting fabric and thread, but of course you'll want to match your thread closely to the color of the dress.
1. The idea is to sew a line of straight stitches in 2 places on the dress and gently gather them. So that the gathers are hidden, start sewing at the underarm seam and sew for about 3" towards the front of the dress. A good guidelines is start at the loops used to hang the dress and end where you'd attach removeable straps in the front (see first picture).
2. Sew a straight line about 1/4" from the top of the bodice. DON'T tie the thread off yet! (picture 2).
3. Pull the thread out 3/4" only (picture 3). Don't pull out any more than this of you will be able to see puckers in the finished product.
4. Tie the thread off by sewing the same stitch in place about 6 times and trim the end; arrange the gathers evenly (picture 4).
5. Repeat on the other side of the dress. This will take the bodice in a full 1.5" and you won't be able to see it when the dress is worn because the gathers are very slight and are hidden under your arm.
Cuteness!