General synonyms:
* Stress
* Highlight
* Accentuate
* Underscore
* Emphasize (yes, it's a synonym of itself!)
* Focus
* Strengthen
* Exaggerate (for more dramatic emphasis)
More specific synonyms:
* Stress (for a strong, urgent emphasis)
* Highlight (for drawing attention to something specific)
* Accentuate (for emphasizing a particular feature)
* Underscore (for reinforcing a point)
* Focus (for concentrating on a particular idea or subject)
* Strengthen (for making a point more forceful)
* Exaggerate (for making a point more dramatic)
The best synonym to use will depend on the specific context of your sentence.