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)