Good Advice from pb and AB. May I also add...
Before walking into an establishment, you must realize that in their 'off-hours,' owners/managers are inundated with requests for face-time by everyone, from wait staff to wine&liquor salespeople. Be respectful of that person's time, and do everything you can to stay memorable in that person's mind after you have left.
Think about what makes you special as an applicant, and what you can bring to the table, and sum it up in a few sentences. Think of yourself as a salesperson and you are selling yourself. Owner/managers don't want to hear about how you've always wanted to mix drinks, entertain people, etc. They want to know how you will save them time/money/bring in business/ make their life easier. Make sure your personal sales pitch focuses on what you can do for THEM, not how this job would make YOU happy.
Use your computer, or hire Kinkos, to create an EYE-CATCHING one page 'resume' with your name, address and cell phone at the top, and a bulleted list (lists are easier to quickly scan, than a wall of paragraphed text) then add some personal information that is relevant to bartending/barbacking. Without experience - give some illustrations/ examples/habits that show you are honest/responsible/a hard worker.
Be truthful. Embellish, but never lie.
Staple a business card to your on-page resume. The card may go in someone's pocket even if the resume gets folded in half and misplaced somewhere.
Don't expect people to call you back. You will be calling them. And again. And again. Until you get a 'no.' Be polite & persistent, but never pushy.
You are your best advocate.
The least attractive attribute of a prospective/new employee is a 'know-it-all' attitude.