1:Must be confident (nervous will let the VO consider you are dishonest)
2:If you cant speak english well,just speak chinese directly, B2 is not F1, You dont have to speak english)
3:Better let your inviter write a letter to VO, explain the reason why you go to US , the letter should tell VO, how tie the relationship between the inviter( even freind could have very tir relationship) and you and you have a good job with decent income in China. You will get back China before the expiration day.
4:The key is let the VO follow the logic that you want to him think.as soon as he believe you, no matrial is really necessary.