What is the best treatment to prevent the recurrence of leg ulcers?

Depends on the cause: Most leg ulcers are the result of poor venous flow out of the leg and chronic swelling. This is not something to self diagnose and treat. Loss of limb is a real possibility if not correctly treated. You need to have diagnostic testing of the blood flow in the legs both venous and arterial perhaps. Frequently the treatment is compression stockings after achieving healing.

Compression: Wearing pressure gradient compression stockings, elevation of your legs whenever sitting or on pillows when laying down and moisturizers on the lower legs.