I check flightaware.com and look a common routes between airports, and simroutes.com. I use Vroute and it will save a flight plan. I do not use FSX flight planner as I find it sub par. I do not use the ATC either. FS has a bad habit of being 5 minutes behind. It has caused some funky issues like a 747 trying to land on top of me! The CS 757 has a far better radar and TCAS than apparently the ATC does!