I haven't seen any evidence that DVT is any more common in aircraft than in any other group of immobilised people. There doesn't seem to be anything special about the aviation environment as far as the formation of DVT is concerned.
Certainly a cramped cockpit does discourage movement, but one can still get up occasionally or have a move around in the seat.
Gingernut has a fair point on the use of aspirin (risk:benefit), but it might be reasonable in a fit person with no significant GIT history, and taken with food.
Long patrol flights in large aircraft shouldn't be a problem. As far as fast jets go, maybe the G suit offers a degree of protection, although it won't be inflated for the majority of the flight. Perhaps they should pull a bit every 30 min or so to avoid any pooling
