Volunteering at a university over the summer

<p>I'll be a senior next year and I intend to major in a science field. I'm hoping to get into a top school, but I need better EC's. I've heard of people getting some sort of unpaid internship/volunteer position at a research laboratory at a university by writing letters to a bunch of professors (rather than through an official, listed internship program). Has anyone done this or does anyone have more information about how to go about this kind of thing?</p>