So I am one for having preterm labor. The only way to pretty much prevent it from happening is bed rest. But i'd talk to your doctor about it more. Did you have a c-section the second time?
I'd say no lifting anything once you got to about 4 months or so. Try to avoid stress. And really try to keep laying down and what not. They may reccommend hospital bed rest with magenisum sulfate from like 6 months on or so while they keep an eye on you!
Both of my labors so far were, just random water breaking baby out! And no reason. My doctor thinks I cook my babies fast, and so one will be interesting. I'm not expected to make it past 35 weeks.
Good Luck
I know that this isn't going to help at all, but when you deliver any baby for early for whatever reason you are defianlty at risk for delivering preterm again. Sometimes there is no answer as to why it happens, it just does. i'm not sure if being on bedrest would help or not, and considering you have 2 other children, i'm sure you don't wan that