DON'T work at a law firm. It really doesnt help your application, as many applicants do that. Do it only if your unsure about going into law and want to know the nuts and bolts of how a firm works.
If your from a wealthy family (or even middle class) do humanitarian work. The Red Cross, Soup Kitchens, Volunteering, etc. If you grew up poor, try and get internships at high-end places like Merrill Lynch or something. This advice isnt just comming out of my arse, its from Columbia's dean of admissions in the book "How to get into the top law schools"
Also, start studying for the LSAT. The most important factor to getting in.