If you have a lot of emails you need to read in a folder, select the first one and then press the space key to scroll to the bottom of the email. Once you reach the bottom of that email, the next email in the list will open, and so on.
To turn this on in Outlook 2007, go to Tools > Options > Other tab > Reading Pane button, and in Outlook 2010 and 2013, go to View > Reading Pane > Options
To turn this on in Outlook 2007, go to Tools > Options > Other tab > Reading Pane button, and in Outlook 2010 and 2013, go to View > Reading Pane > Options