Everyone's medical condition is different, but do you ever take even a baby aspirin at least 30 minutes prior to your flight? It's recommended to help avoid DVT. Along with aspirin, I've read that the best thing, especially for coach passengers (that would be me, too!), is to stay hydrated (water is the best to drink on a flight), and to get up a few times to stretch your legs.
Aside of the above, I agree with other members here who suggested extra leg room seats or business class (at least on most airlines, those two categories can make a big difference for some people--worth the extra cost).
P.S.: For my flight tomorrow morning, I bought a small bottle of baby aspirin the other day. It said cherry flavored on the package. So, I tried one and it was delicious!
