I'm afraid you won't like my advice - just write (not type) more, copy texts etc. Visual memory is supported by motor memory. Sometimes when my mind goes suddenly blank and I'm puzzled over spelling I try to switch off my brains and let my hand write it. It works!
Some people are very intelligent but make awful spelling mistakes. I had a pupil (a bright boy of 15), whose Russian spelling was appalling. Ironically, his English one was excellent. I ascribe it to the fact he wrote a lot in English - besides numerous grammar exercises he always copied some short texts he had to translate: each English sentence was followed by his translation.
I think other members will suggest some less boring methods.