I brought a great pair of white capris at the thrift store but as with most of my pants they didn't fit because my butt is too large and my waist too small, which cause a large gap in the back which gets worse when I sit down.
So what's girl to do... well first I dyed them black but they came out a dark navy blue which was even better.
Then I used my seam ripper and opened up a small amount of the casting in the waist. I threaded 1 and half inch elastic through the casting and pulled it up to fit. Then I hand sewed the elastic in place. And ta-da no more undie action when I stand or sit.