Can anyone give tips how to roll R's?

再见 or 拜拜? Which is more common to say, 再见 or 拜拜? When is each word is used and which situation?

How do you use duì bu qǐ and bùhăoyìsi? Hello. Can someone help explain and give some example scenarios of when would you use "duì bu qǐ" and "bùhăoyìsi"? When is it appropriate to say one and the other? I don't want to say one and have the other person look at me like 'Um.. that's not right'. Please help