|I recently learned this when doing my first pleated dress.
To make the pleat lines hold up during washing, they need to be chemically set. Combine two cups of water with 1/2 cup of white vinegar in a flat container. If you want to be doubly sure, increase the amount of vinegar to 1 cup. Soak a press cloth in the solution. A tea towel or a thin dish towel work very well. Wring out the excess moisture until the press cloth is just damp and lay it over the pleats. Now iron it dry. Make sure that the iron is set on a temperature that's right for your fabric, or you might end up with slight scorch marks on the inside of the pleats. The funny thing though is that the scorch marks will be where the iron didn't touch if your temperature is too high. Repeat until this has been done on every single pleat.
Now, it is best to do this while the basting stitches are still in. After the pleats have been chemically set, remove the basting stitches. I actually ironed it again afterwards to make sure that the pleats are crisp.
This process will insure that the pleat lines stay in through washing. You will need to iron the garment after washing to make sure your pleats are crisp again, but even after three washings with no in-between ironings, the pleats lines are still easy to see.