My company utilizes a two-fold approval system. First the manager in charge of the project/dept budget has to approve the report. By doing so they are indicating that the expenses are legitmate and correct. Then the report is audited by an off-shore team (not Concur Audit) that review 100% of reports against company policy. Ideally they should not find any errors since the report has already been approved by the manager, but given the fallability of our species my team does in fact find quite few errors.
If expenses are not to be reimbursed then the report is sent back to the employee and they are asked to remove those charges. If they used a corporate credit card, then the report is sent back to the employee with the instructions to indicate that charge(s) as a Personal Charge so that it is deducted from their payroll.
Our policy reflects the same as the previous comment. Adding to that, its imperative to remind/educate employees on the T&E policy and it is also their manager's responsibility that new hires are aware. We use expense rejections as an opportunity to re-educate the employee and often provide a one time exception. We also host quarterly calls with the travel arrangers as they are our advocates of the program.
Most Important: Have a clear T&E policy and plan to educate the user.
2nd: Encourage by practice to Plan all trips in advance as much as possible to take advantage of better discount and price for Airfare, Hotel etc which are the major line items. For all trips a pre-trip approval (Via Concur Request) is must to ensure the expense is pre-approved and also the approval workflow has a condition that if the domestic travel is within 7 days or International within 14 days, it requires 2nd level approval - this step discourages Traveller from last-minute booking.
Audit: Although the line managers are technically responsible to review claims and approve, the practical side of experience says they may not check every aspect of the claim and thus an Audit by the auditor is a good way to ensure policy compliance.
You can plan to start with 100% and later reduced the % after reviewing user behaviour, rate of rejection and comments from Auditor.