When it comes to synonyms for the phrase "lie prone," there are a few different options to choose from. One common alternative is "lie flat," which has a similar meaning and can be used interchangeably in many contexts. Other options include "stretch out," "lie prostrate," or simply "lie down." Each of these phrases can be used to describe someone lying face down or in a horizontal position, but their connotations may vary slightly depending on the specific context. Ultimately, the best synonym to use will depend on the tone and meaning that you are trying to convey.