Subject: Lab Volunteering
Hi Dr PI
My name is smukke, I'm an undergrad at whatever-local-university and saw your labs website/heard from X/found-it-in-a-fortune-cookie/etc that you do research in Y. I'm interested in Y because (draw on your vast previous experience, classes you've taken, what you heard from Oprah, the fact that you are a premed, etc but keep it to a sentence, add in a sentence if what you can contribute if unique like I have a MS in statistics).
I'm interested in volunteering in your lab, if you have any openings would you have time to meet and discuss the details. I'm available (keep it broad and general times, like weekday afternoons after 3pm).
Thank you for your time
smukke
Keep it short and sweet. If free labor doesn't get them, they aren't interested.