Why do I see a "under construction" page when I press the Preview button on a campaign?

If your Campaign's Preview button hasn't been setup up just right, you might sometimes see a webpage that says something like:  "...is under construction" with the Force.com logo on it.  Here's a few things you can try to make sure the proper preview of your email appears instead.

1)  Go to Setup > Build > Customize > Campaigns > Buttons and Links > Custom Buttons and Links > Preview > Edit > the big text area at the bottom of the page should have:  <<Your New Site URL>>/apex/sbm__sbx_emailviewbodyhtml?id={!Campaign.Id}

NOTE: Replace “<<Your New Site URL>> with your Site URL, INCLUDING the "/track" at the end of it.

So, if your Site URL is "http://picnet.force.com/track", your URL in this big text area would be:

http://picnet.force.com/track/apex/sbm__sbx_emailviewbodyhtml?id={!Campaign.Id}

Very often, people just forget to include the "/track" before the "/apex..."

2) Ensure that your Campaign's related Email Template does not have a blank EmailBodyHTML field.