I think a fieldworker should be respectful and honest about why they're there, perhaps with statements such as:
"I'm trying to find information about... and wonder if you could help me."
"The information will be used in the following way..."
"If after you have said something you feel uncomfortable about sharing it, let me know and I will delete it; I want you to feel comfortable with our exchange."
"I appreciate your willingness to help me, and as a symbol of that appreciation I'll be happy to send you a photo and copy of the interview; is there something else I can do for you?"
If I were giving one piece of practical advice to a new fieldworker it would be that you should NEVER rely on technology to ensure the success of your work. The best technology is a notebook and a pencil and the researcher should always act as if that is the only thing that will work in the field.
Don't go in with preconceived ideas, or questions that are trying to get verification of what you think.
Do enough background research on your subject matter to know what to ask about, and what information might be new and revealing.
Some of the formalities that older interviewees will expect: saying "thank you for meeting with me/having me over/being part of this project," dressing not-too-casually, turning off the ringer on one's cell phone and not checking messages during a visit, standing up to shake hands when someone new arrives, reigning in the tendency to say "oh my god" when working with people of faith, and remembering to send a thank-you note after an interview.
Eat what you're served, no questions asked. (I discovered once that southern Delaware slippery dumplin's and lima beans made for a very yummy lunch, despite some initial concerns).
Trust your instincts about people. 99% of the people we meet "in the field" are wonderful, trustworthy, fine people, but as with any sampling of the population there are going to be folks to watch out for too. Always let someone know where you're going.
If you are recording something, jot down a time code in your notes so you can find it later, make a note of what you took a picture of when you were at a certain point in the conversation.
Get to "mapping" your materials as soon as possible after you acquire them. Some details and formal description you can do later, but so much of what is important is not the materials you gathered, but your recollection of how these materials fit into the broader world of what was not recorded.
Journaling your impressions of the day can be hugely helpful, because it allows you to record things that happened outside of the fieldwork event.
One of the most common difficulties new fieldworkers deal with is comfort with long silences. if you ask a question and the person needs time to think about it, or if they pause during an answer, don't jump right in with another question. Make sure you give them space to expand on their thought if they want to.
If they ask a yes or no question they are going to get a yes or no answer.
Be sensitive to the manner and pace of the person you are interviewing. The ideal is to interview at their pace.
Keep any promises you make (e.g. "I'll send you a copy...").
Consider the privilege of honoring a person's (life)work/craft/artisanship and be proactive about finding ways to share the information with the larger public.
People are sharing their lives and traditions with you, and you might just end up forming some really strong, long-term relationships. Be prepared to share your future time with the folks whom you have interviewed, to assist them in their projects if they ever call upon you.
Thanks to Betty, Lydia, Theresa, Arle Lommel, Elaine Thatcher, Kelly Feltault, Sarah Bryan, Frank Proschan, Robert, Kiran Sirah, Douglas Manger, Lisa Falk, Susan Davis, Andrea Graham, and other anonymous contributors from the PUBLORE listserv; Lisa McCormick, Brook Lillehaugen, and other faculty at Haverford; and Nathan Moore and other members of Team Folklore for the advice, stories, and recommended resources for new fieldworkers.