It seems that the only way to reduce risk with Rh- is to have an unhindered birth: no labor-stimulating drugs (which can cause small or large disruptions of the placenta, causing bleeds that could put baby's blood into your bloodstream and cause sensitization), no messing with the cord or placenta after baby is out (same reason), and otherwise, letting birth take its natural course without interfering/intervening as long as mom and baby are doing well (as is most often the case anyway).
Some degree of rh sensitization occurs due to birth intervention and invasive pregnancy procedures such as amniocentesis. However, most sensitization occurs accidentally as it were--small or large events during pregnancy that cause disruptions of the placenta's bond with the uterus, allowing baby blood into mom's bloodstream. Being in a car collision or having a fairly bad fall are 2 examples of the kind of known accidents that can (not necessarily, but *can*) lead to placental disruption and blood mixing. There are other pregnancies in which, for no discernable reason, there might be a small placental abruption that repairs itself and is of no real consequence to the placenta's efficiency--but in these cases, blood mixing and sensitization can occur.
To determine if you've ever been sensitized, you can get blood testing during early pregnancy to see if you carry antibodies. (antibody screening) This can be done as well in late pregnancy, and may be advisable because you could have a placental disruption just large enough to cause sensitization without ever knowing it from external signs. After birth, you can test again--this time with a different test done within the first 2-4 days, to see if there are any baby blood cells in your circulation (rosette test or Kleihauer-Betke test). If you do have baby blood in your bloodstream, you can then get the Rhogam shot to prevent your body from making antibodies that would impact the next pregnancy.
