It’s worth considering the difference between the prepositions upon and on (whether or not “on” is preceded by “up”). While for many writers the two have become interchangeable, there are important differences you should remember.
Most sources tend to agree that you should use “upon” sparingly and usually just for literary effect. In other words, the “trap” in this Grammar Trap isn’t so much one of proper vs. improper use, but of readability vs. verbiosity.
Example: I rely upon my friends to move my furniture.
OK, “upon” isn’t incorrect, but it’s overkill since “on” works better. Using “upon” here is the equivalent of using an archaic (and overly florid) form of address — “Thou art wise to avoid using such execrable prepositions” instead of “It’s a good idea to avoid ‘upon.’”
Up on or upon?
Example: I put my dusty old books up on the shelf.
Even when “up” and “on” go together like this, I would stick to two words to avoid the archaic usage.
Use literary effect sparingly. I don’t serve up purple prose to describe spraying for ants, so I would avoid writing, “Spray thy chemical products forthwith upon the preying myrmidons.” I would be more direct, “Spray the product directly on the ants.”
When is the effect appropriate? When you’re borrowing a phrase (like the “placed upon a pedestal” example I began with) or you really want to provide some archaic effect. But if your plan is to write in this century, stick with “on” rather than “upon” in most cases.