Models drift from style rules when instructions are long, vague or buried. Put them in a short, separate block at the top, in the system prompt or custom instructions, and write concrete rules. "Sentences under 20 words, no bullet points, British spelling" works far better than "be friendly and professional".
Add one or two short sample paragraphs in the style you want, because examples beat descriptions. Also state what to avoid. In long chats the style fades, so repeat a one-line reminder or start a fresh chat with the same instruction block. For recurring work, save the block as a template, project or custom assistant. If output still drifts, ask the model to restate the rules before it drafts.