I had a lovely summer dress that Mom and Dad bought during a trip to New York. I loved the fabric, but the top was a very, very tight elastic-like band that was crushing my chest! It was so tight that I couldn't even move it down and pull it off as a skirt.
After a long wait in my closet, I finally decided to alter it. I cut the top third off (this was the elastic section) so that I basically had the skirt left. Then I sewed a seam for an elastic band and ran the elastic through. Added the same straps that were on the dress, and voila! A tank top. It came out well - the only thing I might change later is taking it in at the sides a little bit. Since it was a skirt, it is a rather full top. If I do take in the sides, I'll probably use the scraps the make a tie for the waist. The elastic top makes it comfortable but adds no shape there, so a tie/belt could be good!