Bring a gift. Not something crummy. Something fine: loose assam tea from Harrods, macarons from Ladurée, Charbonnel et Walker hot chocolate, a Diptyque candle, Neal's Yard bath oil, an art book on a hot artist, the latest exhibition catalogue from a hyped show, a handmade drawing (preferably framed)...if you're broke go to a second hand bookstore (e.g. the Strand in New York or the little shops on Charing Cross Road in London) and pick out an old photography book.
Dress up and groom yourself: something tasteful, e.g. trousers, unwrinkled shirt, sweater, tidy shoes. No holes or stains. Above all look clean. Check your fingernails. Disguise bite marks if relevant.
Be as polite as possible. Sit up straight. Smile. Say thank you/ please/ I appreciate that. Offer to carry her bags (not her purse.) Hold the door open. Ask her questions about herself, her life, her projects. Complement her and her dogs. Things my mother likes to talk about: New York, Greece, travel, languages, art, architecture, style... I suggest you read the New York Times and the Guardian (or the online editions) so you have a few things to talk about. Most importantly, at the dinner table, put your fork down when not using it to eat. Whatever you do, do not wave it around to punctuate your thoughts in conversation. She might stab you with it if you do.
If you go out to dinner with her, offer to pay the whole bill. Insist upon it at least once, or be sneaky towards the end of the meal and pretend to go to the loo and discreetly arrange with the waiter to pay the bill.
To make a good first impression, don't talk about yourself too much but present yourself in the best possible light. Show her you're motivated, a hard worker. Before you meet her be prepared to answer questions like: Describe your educational background. Where do you see yourself in ten years? What are your professional goals? What has been your biggest achievement to date? (e.g. winning awards, learning languages, exhibiting, opening your own gallery...) It sounds like a job interview but it'll be worth it, because if she likes you she might introduce you to Larry Gagosian or invite you to a Pucci hotpants party on a friend's yacht.
Obviously, don't mention previous experimentation with drugs and /or unconventional sexual practices, even though she's pretty bohemian.
If you stay in her house, be a thoughtful guest. Be tidy. Ask if she needs help with anything around the house.
After you meet her, send her a letter or email saying how lovely it was to meet her. If it was a visit, send a thank you letter.