There are several ways to draw attention to passages in your writing that deserve special emphasis. I'll start, though, with a few means you should avoid.
First, you should never resort to ALL CAPITALS in formal writing. Bigger Type is also out; likewise boldface. They all come across as amateurish — note how rarely you see them in published prose. Professionals know that they're counterproductive. (Here I'm talking just about the body of text: boldface, caps, and larger type are permissible in section headings and things like that.) And exclamation points have to be used very sparingly.
So what's left? — Italics (or underscore; the two are interchangeable) can draw attention to a word or a short phrase, though even this should be used with some care. Use it when you want to highlight a short passage, but don't resort to it over and over again, or it loses its effect.
The best way to draw attention to particular passages, though, is to construct your sentences to put the important words in the most prominent places. A tip: the strongest position in a sentence is often the end, followed by the beginning. Don't waste the beginning or the end of a sentence — the most important parts — with transitional words like however, additionally, moreover, therefore, and so on. Instead of "However, the paper was finished on time" or "The paper was finished on time, however," save the beginning and end of your sentences for more important stuff like nouns and verbs. Try "The paper, however, was finished on time."
Save the end of the sentence for your most important words.
The important thing to remember is that you should use visual cues sparingly. If you ALWAYS resort to BIG, BOLD, ITALICIZED!!! words, your reader is going to stop paying attention.
