Here are a number of polite phrases used when trying to get the attention or help from people. The use of more formal language is common when speaking to someone you do not know:

Interrupting

It is common to first apologize before interrupting another person, or asking for help if you do not know the person.

Excuse me, could I ...
Sorry, do you think I could ...
Excuse me, do you know ...
I beg your pardon, could you help me? (formal)

Asking Someone to Repeat

When asking for information you may have a hard time understanding in public places which can be very noisy (train stations, restaurants, stores, etc.). Here are phrases commonly used to ask someone to repeat what he has said:

Excuse me, I'm afraid I didn't understand. Could you repeat that? (formal)
I'm sorry, I didn't catch that. (informal)
What? (very informal!)
I beg your pardon.

By Kenneth Beare, About.com Guide

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