Each potential supervisor gives different weight to parts of the
application. Reference letters can provide insight on your
potential as a graduate student beyond what is provided by your
transcript. Get to know faculty members who may write you a letter
by working in their labs or taking seminar courses with them.
Approach them early about writing a reference. Outline the reason
for the request (admissions, scholarships, etc.) and provide
background material they can use (e.g., a personal statement,
outline of research experience, transcripts) - specific letters
are more persuasive. Try to learn if they feel they can write a
strong recommendation and provide a way for them to decline
rather than have them write a lukewarm letter. Remember, many
faculty members write dozens of such letters each year. Find a way
to stand out. The American Psychological Society published an
interesting
article in the Observer (18:5, May 2005) on effective
letters of reference.