i work for a science temp agency that also helps with resume and interviewing skills, i asked them what they thought and they told me that hand written and informal is the best way, so i went out a bought professional thank you cards and wrote a short thank you. I was told and e-mail is too informal and doesnt show much effort, also, my interview was out of state so i had everything along with me and wrote out the thank yous immediately after my interview so i could mail them from that town, they probably got them the next day, and they must have worked!! good luck!