Make sure you are talking to the right person. Especially in a larger
institution find out who will be doing the hiring.  The phone calls, letters
and resumes to the personnel department don't always get the information to
the right person.  There might not be an opening right now, but find out
enough about the museum to know who you want to work for.   Call them. Ask
for an appointment, five or ten minutes of their time to discuss what they
look for in employees.  I have never turned down someone who aked to come in
for advice on a museum career and how to be effective in applying with my
institution (a government agency)