Avoid using many quotation marks and italics; they make a page look fidgety. Also they indicate weakness. If your remarks are not strong enough to stand alone, without words or phrases being propped up by quotes or underlinings, they are no better when so decorated.
A lavish use of extracts from other people's writings is undesirable. As I have said elsewhere, neither the publisher nor the reader is keen to pay for what they can read—and probably have already read—elsewhere.
A pedantic style of phraseology, and a desire to let other people see how much one knows, are amateur failings.
Some beginners go to the other extreme, and adopt a slangy, purposely-ungrammatical style, with the beginnings and finals of words clipped away, and a cultivated slovenliness that they imagine gives a picturesque quality, or an ultra up-to-dateness, to their writing.
But no good work is ever built on such foundations. The first thing to aim for is clarity, and the ability to express yourself in an easy, natural and concise manner, always using the fewest and the best words for the purpose, and employing them according to modern methods.
Improbabilities, misnamed "Imaginative Writing"
Amateurs often lean towards the improbable—calling it imaginative work—partly because they fancy they are less hampered by rules and restrictions than if they take everyday, mundane subjects. Yet—paradox though it may seem—the improbable must be bounded by probability in its own sphere; and imagination must be kept within definite limits and work according to definite forms—else it is no better than the gibberings of an unhinged mind.
Beginners frequently choose the moon, the stars, or the ether as the background for their imaginary characters; or they revel in after-death scenes that are supposed to represent the next world—either of suffering or of happiness. And a favourite ending is something like this, "Suddenly I awoke, and lo, it was only a dream," etc.
Avoid all these hackneyed themes, and obvious tricks.
It takes a Dante to lead us convincingly through the mazes of an unknown world.