One of the most common mistakes students make is that they think that fancy words are better than plain words.  They go running to the thesaurus, even with a simple word will do.  The key to sophisticated writing is not flowery language, it's precision.  You can't just choose a word from a thesaurus because it's listed as a synonym, and just any word will do.  You need to select whichever word has the meaning you want.

As an English teacher, I keep my phone next to me while reading (just in case I need to look up a word--and suggest my students do the same). Authors choose words very carefully. I have always appreciated the technicality of writing (when to write using elevated language and when to write using basic language).

I have to admit that sometimes I feel that some people try to use "big" words to impress others (and not used to denote true understanding). It is almost as if they are writing with the thesaurus open so as to find the most impressive words possible). That said, some writings can discourage the reader when the author makes them feel stupid (through word choice).

Authors need to be aware of their audience and write appropriately. Readers need to engage in the reading and understand that words read are important.

Focus on making your writing clear and concise. Sometimes, new words are not always the best words. At the same time, big, flowery words are not always the best words either. If you write a sentence and find that you can delete whole phrases or clauses from that sentence and still get the same point accross, it is probably best to leave out those extra parts.

Now, instead of adding words, you might want to replace some boring or dull words (or words you've already used quite a few times) with words that are more interesting or give the reader a better visual. For this, I would simply suggest using a thesaurus. I wouldn't overdo it, though, because many readers (especially teachers) can tell when a writer is trying to hard to use "sophisticated," or, as I like to call it, flowery language.

Using new words does not necessarily mean using pompous words. 

You can understand the notion of "new words" in more than one way. For instance, language keeps adding words for new objects or activities that we did not name before for the simple reason that they did not exist. Examples of this are "texting" and "freemium", a condensation of "free" and "premium" used in business to express that a basic product is free while additional, related services are charged. The reason why we cannot speak in dead languages such as Latin is that they lack many of the words we now need for factual communication. 

You can also think of "new words" in relative terms. Words that are new to some may be familiar to others. This depends on the extent to which a speaker of...