One of the commonest flaws I see in writing is words forced into places where they just don't fit.
Often, it's in a character's thoughts: a lot of writers seem to be able to keep speech in character, but when it comes to the character's thoughts it apparently doesn't occur to some people that you also think in words you would naturally use.
Then we come to what I dub Thesaurus syndrome and Mega-verbism.
The former is the unfortunate tendency to look words up in a thesaurus and use the most interesting one. The problem is that if a heading in a thesaurus only contained words with precisely the same shade of meaning there would in almost all cases be only one. Not uncommonly you can find words under the same heading whose meanings don't even overlap. The following should be graven upon your soul in letters of fire:
words under the same heading in a thesaurus are not necessarily interchangeable.
That's not to say that a thesaurus isn't useful: some, perhaps even many of the words will be applicable to the context in which you want to use them, just be sure you know what they mean before you use them. And sometimes, a word with a different shade of meaning is precisely what you are looking for in a thesaurus. Thesauruses are wonderful things, but they should not be used without due care.
The second condition I referred to is my term (and attempt at irony: this is what I find funny) for the compulsion to use the longest, most 'impressive'-sounding words you can, or, having learned a long or impressive-sounding word, to find some pretence to use it. It does not make you look clever. It only makes you look like someone who wants to look clever. And that makes you look less than clever, even if you are clever.
Sometimes, the word that fits is a long one. I'm not saying you should never use long words, far from it - although think carefully if the word is obscure - but you should not be choosing the word because it is long. That will not help your writing in the slightest.
A useful thing to do is to imagine you are not the writer but the reader. Read over what you've written: don't just look, imagine saying or hearing the words. If you've found the right ones in the right places, the golden phrase, it will sound right. If you haven't, you will hear the jarring note.