lifehack.org has a very good posting about how to talk to your professors. I agree with a lot of what is said in this article and highly suggest you take a look at it. The article makes suggestions like:
- Call them by the right title
- Tell the truth
- Be prepared to do the work
- Be clear and concise
- Pay social calls
- Do not, under any circumstances, flirt
- Prepare for disappointment
- Hold the threats
Go to the posting: Advice for Students: How to Talk to Professors
