Personally, when handwriting, I got in the habit of striking a hyphen through the number "7" to disambuate against a possible number "1". Likewise, I'd strike through the letter "Z" to disambiguate from the number "2", and a diagonal strike through the number "0" to differentiate from the letter "O".
My handwriting is terrible, and it has saved me a lot of grief over the years, but especially in my engineering courses in college.