I stumbled upon this thread on Quora and Vitaliy gives a simple yet super effective way to get upsell ideas and to prioritize them. He said:
Make up a list of products you’d like to upsell and assign them a value. This should be based upon your product and your experience selling it. From there on, you can pair products together based on your margins and ultimately what your customers want.
We created our Ultimate Upsell Ideas Spreadsheet as a collaborative tool for upselling and cross-selling ideas.