Creating a cutting for any event is an art. It takes patience, knowledge of structure, and a full understanding of the author's story. Prose is no exception. Do not assume that because you can make use of short story, or a chapter from a book, that cutting a piece is any simpler. Because it's not. This event offers its own challenges to cutting that other events do not. It also offers some of the same hurtles.
NOT a DI/HI Interpretation. By this it is meant that Prose Interpretation is not like Humorous/Dramatic Interpretation in the sense that full-blown character pops are not used. Actually, that level of physical interpretation and back-and-forth dialogue is anti-Prose. Vocalization is essential, as well as interp of the story, but usually through the use of a narrator. Prose pieces that do contain multiple characters generally only use other voices sparingly. Thus, as you cut you need to keep in mind to not set-up a discourse that will eventually become HI or Di-esque. If you are interested in completely embodying and popping between several characters cut a Dramatic or Humorous Interpretation.
Time. Most time limits fall between 8-10 minutes. Your Prose must meet this requirement. The easiest way for a Prose cutting to fit time while doing the least amount of trimming is to select a chapter from a book OR a short story as a piece. However, that does not necessarily mean cutting will be simple. A novel's chapter might need some extra information from previous or later chapters to help create the arc. Short stories might still need to be cut for length. Whichever source material you choose, be certain you can cut it for time. KNOW you can before you begin or risk wasting a lot of effort.
Development complications. Many authors like to take their time developing an idea over a stretch of the piece. And that works in a novel or short story. The reader has the ability to place the work down and continue later. Prose does not offer this luxury. You have to be able to tell a story in time. As you read the ENTIRE material to gather insight for analysis, and to see if the selection is right for you, pay attention to the story. Ask yourself if a tale can be cut down without sacrificing quality. Some ideas are just too grand for the limit time you have to perform. You can always cut an idea to the essentials, but would it make for a good piece when cut?
Find the one story. You Prose should not resemble Spider-man 3's script (a tri-fecta of too many plot-lines and not enough time to develop all of the stories properly). Your Prose needs to have one solid story that is interpreted in-depth. You might be able to slip in a supporting character story if it relates directly to the lead's somehow. Otherwise, read your material and find the one story you wish to perform and make a cutting based around that central idea. This will add focus and a tightness to your piece.
Structure. A cutting is a mini-version of the larger whole. Your cutting must have proper structure or the audience will be left feeling like an element is missing. Basic plot structure is as follows: Exposition (introduction of characters, setting, etc.) --> CONFLICT and Rising Action (the issue is discovered and problems arise due to the conflict) --> Climax (the height of conflict and highest tension; everything is unleashed!) --> Falling Action (things begin to settle down and a solution is sought) --> Dénouement (the resolution/conclusion; things come to an end happily or not). Organizing your cutting around this will help move the plot along and keep the audience involved. If your author tells the story in a non-linear fashion (flashbacks, in reverse) try your best to craft a cutting that mirrors the structure they have created (they wrote it like so for a reason).
Unnecessary information. If you are having difficulties making time you might have to rethink your cutting. Is there any information you left in the piece that has no real reason for being there? Is it absolutely necessary? If the the audience does not need to know then cut that bit until your performance becomes tighter and you have extra time to add it back in. Prose cuttings are organic and changing. You can always make adjustments. Do try to keep a couple jokes or extra fluff though as a way to draw the audience in--if you can. 10 minutes of straight fact or sorrow is tiring. A good laugh is often needed and wanted.
These tips and tricks can help you create a solid Prose cutting. As stated earlier, most of this is useful for doing cross-event work. Yet, there are some particulars unique to Prose. Think logically and design a Prose cutting around what type of piece you wish to practice and perform. What story is it you wish to tell? Answer that and chase after it.