Leaving Alaska for the Lower 48 involves crossing Canadian airspace and therefore an international flight plan is required for IFR flights
You will be asked by flight service to provide just a few extra pieces of information for an international flight plan. First, be prepared to give border crossing times. This means how many minutes after departure will you be crossing into Canadian airspace and how many minutes after departure will you be re-entering US airspace. Your border crossing points must also be provided unless flying an airway. You may use lat/longs or intersections or navaids. These points should be within 50 miles of the border for entering and exiting Canadian airspace and also a point should be specified in US airspace close to the border.
IFR flight plans to the Lower 48 with a departure point, a destination and simply a routing of Direct are not acceptable.
If departing from Juneau you will also be asked by Juneau tower to file a departure procedure.
If you are planning a stop in Canada be sure to make your customs arrangements and advise flight service when filing that you have done this.
Finally, to expedite your flight plan, file at least 30 minutes prior to your proposed departure. During busy summer months this helps everyone, from flight service, to tower, to Anchorage Center, to fit you in and helps you to avoid a ground delay for flight planning.