Details, details: Find out as much as you can about the project and the research environment before you accept to participate. Discuss issues such as how many hours/week can you devote to the project (and is this in agreement with your research mentor's expectations)? Who will supervise you (and are you comfortable with this arrangement)? How often will you meet with your faculty advisor (or will you mostly interact with another laboratory member; is the arrangement acceptable to you)? How will your performance be evaluated? What skills will your project require and if you need training, when can this training begin? Discuss whether you need training in lab safety, use of radioisotopes, animal handling, or whether you need any tests/vaccinations. What is the laboratory protocol for notebook keeping? Will you be working in a project that might lead to a publication?
The above questions are offered as a starting point. In our experience, most misunderstandings between students and mentors stem from a lack of clear expectations on these issues.