To use reciprocity for cross-selling, you need to offer something that is relevant, useful, and timely to your customers. Don't just give them something random or generic, but something that matches their needs, preferences, and goals. For example, if you sell software, you can offer them a free trial of a premium feature, a personalized tutorial, or a complimentary consultation. If you sell clothes, you can offer them a free accessory, a style guide, or a loyalty card. The key is to make your offer valuable, but not too expensive or overwhelming, so that your customers feel comfortable and not pressured.