Tips for Upselling in B2B SaaS: Share Your Experiences!

Happy New Year everyone. One of the things I want to focus on this year is upselling to customers. We are a B2B SaaS company with 4 different products, but most of our customers only are using 1 or 2. Can anyone give me tips or share lessons you have learned from experience? Feel free to share articles but please stick to things you have actually found useful. Don't send SEO fluff.