Say to prospects, "My company will allow me to upgrade you to the superior service or package of services for only a nominal amount more than the good basic service you chose originally. Instead of having to pay two times more to move from our basic to our superior service, I can upgrade you or add it on for a modest amount more." (At this point you specify what the amount is.)
The trick in the upsell is your script. It acknowledges the prudent decision the customer or client has made and is about to sign off on. It then alerts them to the fact that since they've already decided they want that result, they probably would have an even greater appreciation for the superior result - which you can give them for only a modest amount more!
A word of Caution: an "upsell" will backfire on you and breed resentment if your customers perceive it as only crass bait-and-switch strategy. To avoid that pitfall, halt the upsell whenever a customer or client shows by words, gesture or expression that they have serious reservations about upgrading. At that point, you should put them at ease and start talking again about the high value in their first purchase choice.
In any selling, your first objective should be to remove prospect resistance. There are a number of massively effective ways to do that. Each of the techniques is ethical, each is beneficial to customers and clients - and each is non-manipulative.
Risk Reversal: Here, you (as seller) assume most or all the risk in a transaction and your customer assumes none of it. The textbook example of risk reversal is the 100% guarantee of a full refund with no questions asked. It's a wonderful way to sweep away buyer doubt or hesitation.
I believe strongly in "reversing the risk" and, as some readers know from firsthand experience, I use the technique at my business-building seminars. If a seminar attendee doesn't find solid value by the second day of a three-day seminar, I refund their money.
Two Step Selling: Again, the object is to neutralize or remove customer doubt. Two-step selling does that, because in a Two Step you offer your prospects something of clear value, free of charge. The free item could be a free information document, a free lesson, a free trial of something or (if you're a physician) even free flu shots!
Both techniques - risk reversal and two-step selling - establish you as a seller who can be trusted and one who is trying to add genuine value and advantage to a customer's life.