How to clean commercial carpet?

I have the cheap commercial carpet in my kitchen and dining room (not by choice, we are renting). I hate when carpet looks bad. To an average person you may say my carpet looks fine, but I see spots that I want to remove. There are water spots. It is like in the winter when people would come in with snowy boots, the spot just never goes away. I shampooed the carpet twice and it left line water marks that just never went away. Is there a good way to clean this type of carpet for cheap?