How many times do you use the nouns and pronouns I, me or my in your writing?
A simple tip that will make your work appear far less narcissistic: systematically eliminate these words. A quick and easy fix is to substitute second-person references (you, your, yours) for all first-person references. This is an ideal solution if you want to write with a deliberately informal tone. For something more formal, use the third person (he/she/they, him/her/them, his/hers/theirs) instead.
Either way, make what you're writing about the reader, not about you.
This isn't a cure for narcissism, but it does treat one of the key symptoms.