There is always a "slight" chance you will find a good F/A who will allow you to move up once the flight has departed. However, almost every request for an upgrade without using your miles will be met with a kind: Are You Joking?
The main reason for this strict policy is twofold. First, the passengers who are flying first and business class paid 4-5 times the amount someone pays for a coach ticket and the resentment is high. Second, If they do it for you, they must do it for the next guy, and the next guy and the next. Another way to use your mileage privelages in coach is available too. American for example, if you have enough miles with AA and you buy a coach seat, they will block the seat next to you so you have more room to be comfortable. Most people dont even realize this policy exists. You lose no miles and you are a heck of alot more comfortable.