Avoid stock expressions (like the plague, as William Safire used to say) and repetitions. Don't say that as a boy your grandmother used to read to you, unless at that stage of her life she really was a boy, in which case you have probably thrown away a better intro. If something is worth hearing or listening to, it's very probably worth reading. So, this above all: Find your own voice.
Writing is like anything - baseball playing, piano playing, sewing, hammering nails. The more you work on it, the better you get. But it seems to take a longer time to get better at writing than hammering nails.