I keep spraining my ankle, what can I do to prevent this?

Find out if there is. An underlying condition predisposing you to this. Tendon problems or altered foot shape are just two of the things that can predispose you to this. I suggest evaluation by an orthopaedic foot and ankle surgeon. One in your area can be found at www.Orthodoc.Aaos.Org/footankle or 617 983 4886.