This advice comes from a senior undergrad applying to grad school right now. If I was to advise a freshman in university interested in grad school, this is what I would say:
Research is VERY important, since this is what you'll be paid to do in grad school. My advice is to get involved in a research project as early as possible. I know some schools even have a "freshman first research experience" type of program. Look for those. Publications are not required of undergrads (yet

), but they can't hurt. If you can get involved in a very active project and get your name on a paper or two, that will look good on a resume.
Other than research, having great recommendations is also important. Get to know several professors as well as you can, including whoever your doing research with. If they know you better, they will be able to write a more realistic, more personal recommendation that does not sound like it was pulled from a template. This will help your recommendations stand out.
That said, good grades and good GRE scores should not be forgotten. Keep your GPA as high as possible (>=3.5) at the least. Also, make sure you
study for the Subject GRE! Many of the students in my year did not study for it and are hurting because their scores are very low. My advice is to start studying the summer before you take the test.
There are also many extra-curricular activities you can get involved in that won't hurt your resume:
Get a student membership in the professional society of you field. (IEEE, SPS Membership Etc.) It would be even better if you become an officer of a student branch for one of these groups. Get involved. I volunteer as a judge for the Pennsylvania Junior Academy of Science ( an high school science compeition). I also volunteer at another high school physics competition run by my university. Becoming a tutor is also a good idea. It will help you remember introductory material and it looks great on an application for a teaching assistantship.
Anyway, that is probably more than you asked for!