I've read some women take epsom salt baths. You can ask your doctor what they think about it... good luck ladies
I don't think there's anything you can do to prevent since no one knows exactly what causes it.
But, if take preventative measures against the things they think might contribute to preeclampsia... Cut back on salt and fatty foods, light exercises to help prevent the high BP. Severe gum disease is also thought to have something to do with it, so see your dentist and take good care of your mouth. My doctor even suggested I take Emergen-C to help care for my gums. I also read vitamin d deficiency could be a factor, so maybe talk to your doctor about adding some extra d supplement along with your prenatal vitamin.
I've just been diagnosed with preeclampsia so I've been reading about it a lot. Fortunately for me I'm late in my pregnancy so I can just deliver early. I hope you find a solution or at least a way to keep it at bay. Good luck.