The way you emphasize depends on what you're trying to emphasize.
Italics are appropriate in dialogue, like in <PERSON>'s example. If there's a chance a particular word or phrase might be overlooked, make it stand out. You could use lengthy sentences to emphasize panic. There's no mental "breath" for the reader, inducing a subconscious quicker read to get to the break. Use a metaphor to emphasize an unusual characteristic. Use alliteration to drive a point home.
Here's a simple example - I want to emphasize that time is passing agonizingly slow for my character who is waiting for something:
The plastic chair squeaked as she sat down. She checked the clock. The second hand was ticking so slowly. It was like it wasn't moving at all. She picked up a wrinkled magazine from the table to her right and flipped through the pages. She was bored. She put the magazine down. Was the clock even working?
Huh, well, the reader is bored, but not in the way the character is bored. Italics are used for emphasis of the slow time and the character's boredom, but this isn't the right scenario for that device.
The clock read 12:38pm. The plastic chair squeaked as she sat down. 873
The clock read 12:38pm. The second hand ticked. Maybe. She picked up a wrinkled magazine from the table to her right. The clock read 12:39pm. Good. She flipped through the pages. The clock read 12:39pm. She put the magazine down. The clock read 12:40pm.
Here, I used both repetition and short sentences. She's bored, she's restless, she's annoyed. The reader starts to feel the same. What will the time read next? Will it be the same? Different? The short sentences might get tedious to read, but it's piquing the reader's curiosity - what is she waiting for? When will it happen?
So, in essence, there is a right and a wrong way to emphasize something, but no solid list of right and wrong ways to emphasize. Each has their own pros and cons. If you're struggling to find the right path to take, try re-writing your scene over in the various methods people have already suggested in the answers and comments. Then, pick the one that works best (or, if you're not sure, an outside reader can tell you how they felt in each version). 487