How to keep No1 in Google's Adwords

The reason for the drop of your ranking is because some other companies who also bid your keywords have increased their budget per click.
I think there is an option inside Google Adword to bid for position and adjust the budget automatically in order to secure the position. Personally, I don't prefer this option because it is easily manipulated by some people, and the budget is more prone to out of control.
So, you can somehow understand why Googles has become no. one profitable in the world because people compete to be number one and pay more money. The system ultimate benefit Google.
If the keyword you are using is rather expensive, you can think of other keywords. The more you creative the more you could reach success.
By the way, rank no. one does not mean anything. The result is also related to your web site; how it is designed and organized; the content etc.
There are many good management tools inside Google adwords. YOu have to study it in order to fully utilize service. There is one useful tool to track how the click is finally related to purchase in the website. This may involve some homepage coding, but its good.