Sign by the X please

Just wondering how to write up a contract for web page or even logo design… I am requireing a %50 deposit as of now and %50 when done but the problem i am running into is getting content from the customer they drag thier feet and I need to get the project done and start a new one… what is a tactfully way to say i need your content by this date or you need to pay and ill get back to you when you are ready to do soemthing