Many times, Front Desk receptionists find it daunting/awkward/difficult to carry out upselling for arriving guests. They are right in thinking that guests usually can't wait to settle in their room and any further delay in the check-in process may lead to a less than optimal arrival experience.
The key is to understand and put oneself in the guest's shoes. You can do that with the booking information on hand, but it is not likely to be sufficient for you to know more about their needs.
Ask, to show interest about them, their stay and their expressed and unexpressed needs. (Note that these should be executed while multi-tasking, so that there is no unnecessary delay for the check-in. When done well, such engagement should not add further time to the overall check-in process.)
Make a mental "match" and suggest if there is an alternative arrangement that will suit or entice them more than their original booked room category. Recommend if there is something you feel is worthwhile for them, highlighting succinctly how they can benefit from it.
Be natural, not scripted. If you look hesitant and feel like you are troubling the guests, they will feel it exactly the same way. If you demonstrate genuine interest, care and extend warm hospitality, they will be more receptive to what you have to say and suggest. You are what you project.
At the end of the day, more than "just a receptionist handling the check-in", you are a hotel ambassador who helps the guests to better enjoy their stay with you.
If you want to be a sales person, sell.
If you want to help others, don't just sell but help.
Hence, I suggest we call it upHelp and no longer upSell.
Reflective thoughts: What are some "not-to-do" for upHelp? What would put the guests off?