Tutorial

This shirt used to be light green and I bleached it to have a basic undershirt type layering thing going on. Anyway, to add more interest I wanted to put the elastic like I explained before and thought I'd just take some pictures to make a little tutorial just for fun.

I cut the elastic about 4 inches long (this can very on each person and top, but make it about as long as the distance between the point of the V and almost to the bottom of your bra.) Pin the top of the elastic at the point of the V on the WRONG side of the shirt.
Stretch out the elastic straight down and pin it. Depending on how much you stretch it you will have more or less gathers.. it all depends on what you are going for and you can carefully cut the threads and try again with less or more gathers if you decide you wanted it slightly different after trying it the first time (I actually did this twice while I was making this "tutorial") A good seam ripper is the most important tool for any seamstress, and the people who are good at sewing are really only those people who are willing to rip out and do it again. Only after years and years of practice can things be sewn with less ripping out.. and it still happens. Anyway, it's normal so feel free to change stuff and try again!
When you go to sew it, make sure your bobbin thread and top thread match your shirt (the bobbin thread will be what shows on the outside... I neglected to change the bobbin thread and had to take out some blue and change it to the same color as the top thread... lame.) Stretch out the elastic while you sew a straight stitch. You'll have to pull with one hand behind the needle and foot and pull the elastic with the other hand. I use my left hand in the front and my right hand in the back, but I've been known to switch hands so just go with which ever one feels the most comfortable and you have the most control over the fabric. Always do some back stitches at the beginning before pulling the elastic and do some back stitches at the end as well.
Cut the threads, turn it right side out, and viola! A shapely waist slimming style.

I'm going to do this to a turquoise one that I have too.. and maybe add beads to that one as well.. we'll see. :)