What I’ve seen being done before, not only by lefties but actually by quite a few people, is to shift your paper 45~90°, so that you’re effectively writing bottom up (right handed) or top to bottom (left handed). It can get a moment to get used to it but it alleviates the smudging significantly.

For what it’s worth, personally, I don’t like it so much, but I know people who swear by it; and had fast, clear, legible notes to back it up.