Another very good control to have in the accounts payables process is that employee expense reports should be reviewed and approved by someone who is not the owner of the expense report - this goes for company founders and C-level executives as well. This review needs to be formally documented to pass Sarbanes-Oxley.
A great way to accomplish this is to use an employee expense report form. We have a great employee expense report that we'd be happy to give to you. Please send an email to firstname.lastname@example.org and we'll send you a copy!