You need to study the published route (or routes) from the A/FD in detail prior to getting into the airplane. Make sure the airplane is equipped to handle the route and that the route applies to the correct aircraft classification (jet - J, turbo prop - M, non-jet 190 knots or greater - P, and non-jet 189 knots or less - Q). No IFR flight plan needs to be filed with the flight service station or DUATS. You can simply call clearance delivery or, if clearance delivery is not available, ground control, and request a “tower en route” or “tower-to-tower” to your destination airport. Just remember to give the controller your airplane type and suffix on the initial call. The controller will then issue your IFR clearance, and you’re good to go.