Also a grad student, and with my family and friends who are non-scientists, I also refer to myself as a teaching assistant or instructor. Because that way they'll ask me interesting questions about what it's like to deal with students and if I enjoy teaching and the like. It's a required part of my program some quarters (all depends on the NIH funding!) and while it's not the part of the program that captivates me the most, it's much easier to talk about. If I lead off by explaining that I'm a grad student doing a research project, then they'll ask about what I do and I'll spend the next 10 minutes trying to explain things in layman's terms and their eyes still glaze over. It's part of being an interesting conversationalist, being able to talk about yourself in ways that are actually interesting to other people. (Which of course is a corollary to being interested in other people and being able to draw them out with questions about themselves.)