Just a personal prefix that I like to use rather than "usp_" is "up_" this way I can perform a find for every occurance of "up_" to find all user procedures in a large script, "sp_" to find all system procedures in a large script and "p_ to find all procedures, user or system, in a large script.

But my personal preference for any standard naming conventions is to simply have one and have it documented. Unless Microsoft decide to set it in stone as part of the SQL code, no-one will ever agree on a common convention across the board but as long as we have a convention and have it documented this can make it all the more easier for outsiders or fellow co-workers to navigate our work and save far more time than it costs.