What's considered the best way to help prevent me from getting deep vein thrombosis or blood clots in the legs when I get older?

Routine Activity. Routine exercise and activity will help to prevent stasis of the blood in the leg veins, one of the predisposing factors for dvt. See your primary doctor for routine yearly exam, and if you have a family history or risk factor history, further testing may be recommended. Varicose veins, swelling can be helpful external signs of venous insufficiency which can be associated with dvt.
Mobilize. Reduce risk factors walk or proper exercize weight to ideal. Ask your pcp about any clotting disorders. Probably avoid birth control meds.