dehydration and sitting without moving make one prone to blood clots.
For example, the BBC had a couple health columns last year about long airline flights and blood clots.
http://news.bbc.co.uk/2/hi/uk_news/england/1886083.stm One way to decrease the risk would be to have all soldiers take an aspirin every day. However, if they were shot or in a car wreck, taking an aspirin would increase the amount of bleeding from wounds, and so it would probably kill more people than prevent deaths from DVT
If you are sitting, support stockings (TED hose) also prevent clots, and I usually wear them on long flights. However, they are hot and uncomfortable, and if you bend your knees they wrinkle. So again not practical for combat. But helpful on long flights.