Well since I have one boy and one girl I sell them when the put grow them. After I had my first I saved all her clothes since I wanted to have another but my second was a boy so I could only use a few things I saved. But since you have two girl I would save the oldest clothes for the youngest to wear when she is big enough. And since you want more kids maybe just save some of your favorite things for later.
My two oldest are twins (boys) and wear pretty much the same size and my youngest is a girl. We are finished having kids, unless God sees fit for us to have more since I had my tubes tied after my daughter was born.
Save! My fist ds is 7 and my second ds is 2. I put everything the first outgrows that is still in wearable condition in a box with the size on it so I'll have some things for my youngest when he gets there.
We save them for the next child because I have two boys and two girls so everything comes out evenly. Then my youngest (who are twins)two have twin cousins that are ten months behind them and a another cousin that is a year and two weeks behind them.