If you're offered the opportunity join a lab as an observer, know that it's important to be genuinely enthusiastic and work hard starting on the first day.

Many labs offer an observe to learn position instead of putting undergrads on research projects right away. Although officially it's done to let the student determine if they would be happy in the lab, at the same time the other lab members are deciding if the student seems like a good fit.

Be polite. Be helpful. Be observant. Ask questions about the science the lab does but don't offer insincere compliments to your labmates (it won't help and might hurt your chances).

If you only remember one thing let it be this: the first few weeks are part interview--for you and for the lab.