Though your business is driven by your out-of-the-box ideas and your top-notch execution, don't let the accounting fall by the wayside. Customers are under pressure to stay in (or under) budget - and this is where you can really set yourself and your company apart.
Make sure your team has a clear understanding of what the budget is, how it is allocated and how expenses are going to be tracked. If your non-invoiced payments are usually executed with petty cash, be sure to have a system in place for collecting receipts, recouping excess funds, and accounting for it all (Quicken or Excel are both easy to use options for this). Centralizing petty cash with one person in charge is a must. If it's an event with many distinct departments consider giving each a petty cash fund and an officer to manage it.
If you prefer not to have all that cash roaming around, and want to save yourself some time, trouble, and paperwork try using a controllable corporate card, like PEX Card. PEX Card allows you to set specific spending profiles to each cardholder. For example, one employee may be authorized to spend $100 per day at only retail stores, while another is authorized to spend up to $2,000 in any merchant category. At the end of the event you will not only be on budget, but you'll have a complete record of all spending.
If you are able to stay on or under budget your clients will remember you as that fantastic event marketer with a head for numbers, and you'll be working with them again sometime soon.