If you want to use a preheader in your Exact Target email send (you should), then you’ll attempt to implement the %%_PreHeader%% personalization string listed in their documentation, only to discover it’s inclusion prevents you from doing a “test send” or a “send preview”.
The solution is the following AMPScript, place this in your template:
%%=v(@Preheader)=%%And this in your email properties:
%%[
VAR @Preheader
SET @Preheader = "This is my preheader text"
]%%Note, just like your unsubscribe and web version links, this preheader will not be rendered in a test send, but it will render in a send preview, and of course a real send.