I think that Rape Shield Laws are a start to the long process of allowing victims of sexual assault to come forward and have their voices validated. The "blame the victim" approach is minimized with shield laws that limit the scope of how much scrutiny the victim can be placed under. It is a serious attempt on the legal end to allow the victims' voices to be authenticated.
However, I think that the solution, if there is one, to redressing the under-reporting of sex crimes is to attack it on social and psychological grounds, as well. I don't think this problem is going to reside in one domain, such as the legal realm. It has to take a concerted effort from as many ends as possible to ensure that victims feel comfortable seeking justice through the proper channels. This will involve coaching and assisting, allowing individuals of such violence to not see themselves as victims or as individuals who "asked" for this. Such an endeavor will lie in many realms, and not simply in the legal sphere.