Approvals should be avoided as much as possible in the accounts payable process. There is a significant bottleneck involved in waiting for a manager to approve an invoice, so use as many other alternatives as possible. For example:
Use purchase order as approval. If the purchasing department has already issued a purchase order, then the purchase order itself should be sufficient evidence that an invoice can be paid.
Eliminate approvals for small amounts. Establish a threshold invoice amount, below which there is no need for an approval.
Use negative approvals. Send an invoice copy to an approver, with instructions to only respond if there is a problem with the invoice. The accounts payable staff will assume that all other invoices have been approved by default.
Obtain approvals in person. If it is absolutely necessary to obtain an approval, have an accounting person hand-deliver the invoice, answer any questions posed by the approver, and bring back the signed invoice. Doing so is time-consuming, but ensures that invoices will be returned in a timely manner.