When you save a flight in the PMDG 737 NGX, 6 files are normally created, all preceded by the name of the flight you picked.
From My Documents/Flight Simulator X files:
- a FLT file
- an FSSAVE file
- a WX file
From PMDG\PMDG 737 NGX\PanelState:
- a RTE file (will not be there if there's no active route in the FMC)
- an FMC file
- a SAV file
To send someone your saved flight, you'll need to zip up those 6 files. The person on the other end also needs to have the same livery you were using installed.