It may be conceited for me to write this, as I am no great writer myself. I am, however, a great reader and thought that I would share what I've learned over the years.
1) Avoid adjectives that end in 'ly'. It's a short cut to a description and gets repetitive very quickly.
For example: "He left before he had the chance to think better of it." instead of "He left quickly".
2) Avoid 'some' or 'very'. They are unnecessary words or can always be replaced with better.
I tend to ignore over when reading and remove them completely from my internal dialogue.
3) Don't repeat the same word, or variations of the same word, too closely together. Again, it's repetitive.
Thesaurus.com is my best friend.
4) Avoid using 'said' or other dialogue tags. Instead try to show who is talking through action and description.
For example: "the words left her mouth no louder than a whisper." instead of "she whispered". (Using said is supposed to be the "correct" way to show dialogue, so this one may only be personal preference)