All of my answers are ludicrous and too long, so you can skip to the conclusion at the end if you don't want to read.
I think it can be quite easy to make sure your sentences sound gorgeous and flowing, perfect and natural. I think it would be a good idea to begin by saying this: you do not have to avoid any words or phrases, as long as they fit your style and tone. You should use words that fit the genre and target audience of what you are writing. Be consistent with these and use them throughout the piece, as tonal shifts can ruin your work.
My advice and points
Here are some notes on words however, that you may want to pay attention to:
- Avoid repeating words like and, which, because and other connectives in a sentence. Also avoid repeating subject pronouns constantly.
For example, let us look at the following sentence:
I ate bread and drank milk, and then I went to bed and got to sleep instantly.
This sentence is very poorly written. It uses and three times. Your English teacher will tell you that repetition can be a good technique... But in this case it definitely isn't. It makes your writing appear sloppy, unprofessional and bad. Also, pay attention to the style I wrote this in. I wrote this as if it were a diary, or blog, or retelling of a normal event. Because this is contemporary it would be fine for me to say got to sleep or went to bed . If I was writing fantasy, I would perhaps refrain from these terms which in my eyes, seem too contemporary. Perhaps if you were writing fantasy set in a cliche medieval England setting, you could say fell into a slumber or snagged my eyes shut. But of course, you wouldn't use these terms just around the house. They're very stylistic and revolve around genre and tone very much.
I would rewrite the sentence like this to avoid repeating and and I.
After enjoying bread and milk I went to bed, falling asleep instantly.
We can see that the first part is all a main clause and I am just adding falling asleep instantly as extra information without repeating anything unnecessary.
- Try to use different sentence lengths
Okay, I bet you already knew this, but I'll point it out anyway. You must vary your sentence lengths if you want to have good sounding sentences. Part of making sentences sound good depends on the passage they are in. Of course varying sentence lengths can make for epic cliffhangers:
Go now, she thought, and closed her eyes; and when she opened them again she found she was alone. That was how she learned to do it. ~ Justin Cronin... The Passage.
I swear, that sticks in my head. I can never unsee some of the things I read in that book. It makes for a great example of sentence length being used to create a cliffhanger here.
- Don't use too many interjections
In your narrative not your dialogue try not to use too many interjections. This'll remove a lot of formality, and also make the piece appear like a diary entry. Unless you want your narrator to be like this, don't write something like the following:
She put some socks on; I guess she was cold
The I guess is completely unnecessary here and the narrator doesn't really need to say it.
- Understand English grammar and PUNCTUATION!
This is totally self explanatory. Its so self explanatory I won't bother to extend my explanations even more on it. Good grammar and punctuation = instantly clearer meaning, instantly better sentences, hugely engaging passages. Learn about clauses, stressed syllables, trust me it'll help.
Avoid repeating words like and, which, because and other connectives in a sentence. Also avoid repeating subject pronouns constantly.
Try to use different sentence lengths
Don't use too many interjections
Understand English grammar and PUNCTUATION!
Pick a style and be consistent with it throughout the entire thing. Ensure that every word is good for your genre and audience.