Don't start your question with the phrase "I hate to bother you..." I don't mind you bothering me- I'm here to help you. It's the deal I made with my employer. Don't worry, I'm compensated for helping you. Also, lets face it: If you really hated to bother me, you just would not unless things got desperate. Don't be insincere.
Do tell us when you think we've been extremely helpful or when you find something you like at the library. We really care about what you like as much as what you don't like.
Don't (and this is a big one) leave your small child unattended anywhere in the building while you go to the restroom/check your e-mail/mess around with your phone. This is a public building. Anyone can, and does, come through our doors. That includes predators. And although most of us are woefully overqualified to watch your child, we are not a daycare.
Do be courteous of other people who are also waiting for help.
Don't be afraid or embarrassed to ask us anything. We really have heard it all, and some of your questions are ones that we hear several times a week.
Do take your phone calls to the lobby where you won't disturbs college students trying to study, homeschool families, people writing resumes and looking for jobs. It's distracting the first time when your phone goes off with its ring tone playing "New York, New York" and it's worse a second time when we listen to you shout "What? WHAT? YEAH, I'M AT THE LIBRARY."
Don't forget to use the library and remember that we're more than just books. We're also a free source of entertainment, a free place to bring your family, a free place to take classes, and a place where information professionals can help you find out about anything. For free.