Email is very unprofessional and inappropriate if it is the only communication you make. Remember that declining an offer doesn't have to mean burning a bridge. You never know when you may cross paths with that firm in the future. I was told to call the hiring partner (don't just leave a message) and then, after you tell the partner that you are declining, email the recruiting coordinator to confirm (just in case the partner doesn't pass the info along). This seemed to work well.