My daughter just started her PhD program in Biological Oceanography. During the summer between sophomore and junior year, she volunteered at a marine lab in Northern California. She contacted researchers who did work on the topics she was interested in and asked if they would take her on as a volunteer. The experience she got that summer was invaluable. Her mentor was so happy with her work that she wrote a great recommendation and was instrumental in my daughter being accepted as a summer research fellow at world class oceanographic institution in the Northeast the following year.
I second the suggestion to look at NSF funded REUs. My older daughter went that route, and did REUs for two summers. She is currently finishing up her PhD in engineering.