How can I output records/messages to a flat file from inside a SQL Server TSQL script/stored-procedure/trigger?

Use xp_cmdshell and the ECHO command. Use the > or >> redirectionsymbols to either create or append to a file.
xp_cmdshell "@ECHO test message >> C:file.fil"

Put the information you want into a table (note this can't be an ordinarytemporary table, but it can be a global temporary table) and then bcp it out toa file via xp_cmdshell.
xp_cmdshell "bcp .. out c:file.fil-Usa -P -c"

BCP or BULK INSERT (SQL 7 only) can also be used to read in a flat fileinto a table, from where it can be processed.

Write your own extended stored procedure. As this is a C program it can usestandard file access commands to achieve whatever you want.

Run the select through ISQL via xp_cmdshell and use the -o parameter tooutput the results to a file. This example uses the -E parameter to avoildhard-coding a userid.
declare @str varchar(255)
select @str='isql -Q"select * from " -E-oc:file.fil'
exec master..xp_cmdshell @str