In Sql Server Management Studio, script filenames end with .sql. For sqlcmd scripts, I use .sqlcmd.sql to distinguish from a transact only .sql script.

create_database.sqlcmd.sql

The above script might call a number of other sqlcmd scripts as well as some transact sql only scripts.

create_database_procedures.sql

In sqlcmd, I precede all script variables with var_. Whenever possible, I use the provided sqlcmd utility script variables. That way the scripts can be called with commandline switches and people can leverage their knowledge. The sqlcmd utility script variables are defined here
http://msdn.microsoft.com/en-us/library/ms162773.aspx

:setvar sqlcmddbname "my_database"
:setvar path_scripts "C:\scripts"