My father has DVT and it's the reason he can't ever visit me on the other side of the planet. He'd have to go to a doctor to get a shot to thin his blood or something like this otherwise he's running danger of exactly what you're describing.

I personally got these leg assisting stockings that my doctor told me to use in long haul flights, and carry aspiring with me to thin my blood. I also try to regularly stand up and walk around