How to improve interview skills- my biggest weakness_:'( _:'( _:

I just went an on site intervew and I think I failed. The good thing is I dont like the position that much.But still, I definately need to improve my interview skill. But how?

Often times when people ask me how to you do this? My answer would be like" I can do this but I dont know how to describe?" This is my biggest weakness.

Does anybody has same problem as me? How do you overcome during the interview?

I have another onsite interview coming next Friday. That's the one I really like it. I dont want to fail.... Please help me....
_:'( _:'( _:'(