Pace
Not an expert on Venous ThromboEmbolism (VTE) but from the knowledge that I do have, managing the risk is better done physically than chemically. Obviously, in smaller aircraft, some of this is a little more challenging than in airliners but here goes...
Maintain hydration - multiple benefits here but part of it may be a reduction in risk of VTE. Certainly, the increased requirement to go to the loo helps with...
Keep mobile - one of the causes of venous stasis in the legs (where nearly all Deep Vein Thromboses (DVTs) originate) is immobility, especially if the venous drainage is further impeded mechanically. Pregnant people have a large mass pressing on their pelvic veins which causes them increased risk but just sitting (especially if you are cramped) increases the obstruction to venous outflow and increases the chance of DVT. Sitting cramped and immobile for a long time (imagine a cattle class flight to the West Coast of the US for instance) is especially bad. Try and get up and move around. There are various devices available that may or may not work (graded compression socks, pillows you 'knead' with your feet etc). I appreciate that moving around a small aircraft is nigh-on impossible and I am afraid I don't know enough about how much rudder action your legs get (I suspect not much at all if you have trimmed the a/c well!) on longer flights but trying to keep your legs moving every so often will certainly help.
Avoid constricting clothes. These can increase the obstruction to flow. Wear socks without tight bands at the top. Consider removing belts from trousers and/or undoing the top button. If you have imprinted bands on your skin after flying, your clothes may be too tight there!
As I have said before, I remain to be convinced that for people with a normal risk of DVT (no predisposing medical condition), aspirin makes any real difference at all. LMWH (see other comment) certainly reduces your risk of DVT but the other advice probably reduces it as much, if not more.
The pressure effect of higher altitude is likely to make a very small difference to the risk of DVT I would have thought but am happy to be corrected by a more qualified aviation physician.
It's called economy class syndrome for a reason. I haven't heard of many aircrew getting DVTs. It's the pax that seem to get them!
Happy flying
BtD
PS...if you're flying unpressurised fast jets then you should be fine. The pumping action of the G-trousers should mean that the blood flow is kept good!