First of all, congrats! We will all be hoping for a totally PE-free pregnancy this go-around.
JenH gave a great response, but just to add to that, the U.S. Preventive Services Task Force (USPSTF) published a review last April talking about the relative risks versus benefits of aspirin to help lower women's risk of preeclampsia (http://www.preeclampsia.org/the-news?id=326
). Generally speaking they found that the benefits outweighed the risk, and found that there was no statistically significant impact on risk of placental abruptions, postpartum hemorrhage, or miscarriage to the mother.
And yeah, we wish there was an easy way to prevent preeclampsia, but hundreds of studies of calcium, protein, vitamin D, vitamin C, and other supplements found no real benefit to a population with good nutrition, like most of the developed world. :/ But the studies keep going, and we really are at a much better understanding of the disease even than we were five years ago! So hopefully we will have more treatment options and preventatives down the road.