@13thfloor IMO, undergrad research programs are looking for these attributes: 1) a strong, expressed interest in research, not just padding a resume for med school; 2) good grades in science courses… usually upperclassmen have an advantage here; 3) external validation (via recommendations) of an interest in science. Previous research can be a plus, especially if you can get a good recommendation from the mentor. On the other hand, some programs like to give slots to those without experience to “spread the wealth”.
It should be understood that having undergrads in one’s lab for just a summer can be an altruistic endeavor on the lab’s part. It takes time for people to become productive and effort to teach the undergrad. It’s all about data generation for the next grant/publication. If you can find a research lab close to your school such that more than a summer or semester can be devoted to the lab, you will have an upperhand in securing a position.
As another poster stated, application deadlines for summer research programs occur around this time of year. If you happen to miss the deadlines, there may be opportunities outside of formal programs. Contact the department heads or lab heads directly via phone/email to see if they might take a student on for the summer… this works best if you have a personal or mutual connection however.