Don't deny the trip.  Once you deny the trip the requester cannot add the missing information.  Ask for the additional information by doing the following:

Scroll near the bottom of the trip request and Send Email:

Requester email address will populate when you click the button "Add Requester to 'Send To' Area" button.  Enter a subject (it's best to use the trip number as your subject) and your message and click Send Now.  This message will be logged in the history at the bottom of the request form.  A copy of the email will also be retained in the Email Audit Log.  Ask the requester to send the missing information and then open the trip back up and approve/deny the trip.  

Logged in the History:

Copy of email is in the Email Audit Log and may be viewed by clicking on the envelope next to Email Sent By: