From a Production Standpoint
Practicality is more important in writing a short film script than in feature screenwriting because a short film script is not usually a piece of marketable material. Feature film scripts can be purchased by studio but never made, sold as books, or act as the inspiration for another work. Short film scripts are almost never given the same credibility, so the purpose of them becomes much more plain. They exist to be made. In this way you need to write the short script while thinking about actually making the project.
First, you need to consider the overall length. Given the standard screenwriting equation that a page means about a minute of screen time, you have to consider how long you want your short film to be. Short films that have too long of a run time, which usually means more than twenty minutes, have difficulty at film festivals and often have more difficulty getting out. Very short screenplays, fewer than eight pages, have a difficulty getting anything done unless they forgo narrative structure and take a more experimental approach. Between ten and eighteen minutes tends to be a good length for short narrative films.
Next, you have to consider the budget of the film. Difficult locations, special effects, an excess of props, and other production elements can end up costing too much to be practical on a low budget. Since most short films are unfunded, low funded, or student films, it is hard to maintain a script that has things that are financially prohibited. The choice is then not to eliminate these elements, but to go through your script and identify the expensive items and think about why you chose them in the first place. Once you can identify the key reason why they were important you can usually find another item that will meet the same need for a smaller cost.