Simple and Direct:
* "He gagged, a wave of nausea washing over him. He stumbled to the edge of the balcony and emptied his stomach."
* "The food she had eaten came up, a bitter, acidic torrent."
* "He retched, the sound like a strangled animal, before a thick, green fluid erupted from his mouth."
More Descriptive:
* "A hot, acidic sensation rose in his throat, burning its way up to his mouth. He choked, his face contorting in agony, as he spewed forth a tide of yesterday's dinner, the smell of which clung to the air like a noxious fog."
* "The vomit rose in her throat, a thick, yellowish sludge, accompanied by the acrid tang of bile. She gagged, her eyes watering, and the contents of her stomach exploded onto the tiled floor with a sickening splat."
* "He lurched forward, his head spinning, and threw up violently. The vomit streamed down his chin, staining his shirt a sickly green, as he collapsed to his knees, gasping for breath."
Focus on the Emotion:
* "He felt a wave of shame wash over him as he retched, his body betraying him in the most humiliating way."
* "She wanted to scream, but all that came out was a choked gag as she emptied the contents of her stomach into the trash can."
* "The world seemed to spin as he fought back the urge to vomit, a terrifying feeling of helplessness gripping him."
Remember:
* Consider your audience and the tone of your story. A graphic description might be appropriate in a horror novel, but not in a children's book.
* Don't overdo it. A few well-chosen details can be more effective than a lengthy, graphic description.
* Focus on the effect of the vomiting rather than just the physical act itself. How does it make the character feel? How does it impact the story?
Ultimately, the best way to describe vomiting is to find what works best for your story and your writing style.