What are some common foods and herbs that help prevent blood clots?

Naturally thin. Herbs and supplements that thin the blood (and therefore shouldn't be used before surgery or with prescribed anticoagulants) are birch bark, cayenne, cumin, evening primrose oil, feverfew, fish oil, garlic, ginger, ginseng, gingko, grapeseed extract, milk thistle, onion extract, st. John's wort, tumeric, and vitamins c and e.
Aspirin. Aspirin is a very old and well known blood thinner. Aside from medical issues that can cause blood clots, be sure to keep your legs moving during prolonged trips.