Simple messages to the user from a plugin

I’ve made a bunch of wordpress plugins but I still can’t seem to figure out how to display a message just once only using a plugin (no theme editing).

Lets say I do some $_GET and $_POST checking with the ‘init’ action hook, then want to display some text, in a variable $output, after those checks. If you simply echo $output inside of the function init calls, IE renders an error because all the output is before the <!DOCTYPE tag.

I can’t seem to find an action or filter that I can pass $output to so that it displays the output only once, outside of the loop, after the header information. Any ideas? Thanks…

<—header—>
<—my message—>
<—the loop—>
<—other included files—>