Negative prompt

Image diffusion models accept two prompts: positive (what you want) and negative (what you don't). The negative prompt is where most quality fixes happen.

Standard negatives everyone should use: extra fingers, deformed hands, bad anatomy, duplicate, text, watermark, low quality, blurry. Beyond those, add things relevant to your specific failure modes. If the model keeps giving your character the wrong hair color, add the wrong color as negative.

Overloading the negative prompt degrades the positive prompt's influence. Ten items is about the limit for most models before the output starts looking flat or generic.