As long as it flows well that's what really matters. I spend a lot my time reading back what I've wrote out loud. The worst thing is when pretentious people throw in all these complicated words they've found on the internet to make themselves look more literate than they are. There are some genuinely beautiful words out there though. This is going to sound super lame but, I have a blast on dictionary.com looking at all these mental words.
Simple and clear does not equal primitive. Think Hemingway. As has been mentioned before, using a thesaurus and just interchanging words doesn't work. Although I think dictionaries by themselves can be useful, I found I picked up interesting and new words a lot simply by reading good books from a young age. I'd look up words I wasn't familiar with, and so increased my vocabulary. Often when writing, I was able to draw on that knowledge, sometimes years down the line.
