Identify the essential resources, what you can and cannot do with current resources and communicate the gap. Present a clear picture to the decision makers including the results you will be able to generate with the investment.
It is very important to link any resource request to a result. If you can also provide a mechanism for monitoring that result it will make your request more believable. Make sure you are fully maximizing your existing resources prior to asking for new or additional ones. Be able to prove this effort as you make your request.