How to contact a prospective supervisor for undergraduate research at another university?
Some details to include


Attach a copy of your CV and your unofficial transcripts.
Briefly describe your past undergraduate research experience.
Briefly mention the coursework you have taken/are taking that is highly relevant to the work he is doing.
Briefly explain what excites you about the work in this lab.
Mention your future plans (for after graduation). For example, if you are planning to do a PhD, I would be much more interested in mentoring you as an undergraduate research student, because if you are a strong student I can try and recruit you into my PhD program. This is especially true if I am a new professor looking for graduate students!


From the professor's point of view, mentoring a short-term undergraduate student is a relatively low-risk and low-commitment endeavor (compared to taking on a graduate student.) On the other hand, it does require significant investment of time that may never really pay off (the student will probably move on before they become really "useful" to the professor.) Some professors do it largely because their department wants to provide undergraduate research experiences to their own students. In this case, they probably won't be interested in outside students. Some professors do it because they enjoy it (I do!) or because it looks good on grant proposals (it does!), and will be more open to undergraduates from outside their institution. It's certainly worth a shot.