Google has recently rolled out some improvements to the service, and Narayana Tummala at the Inside AdSense blog talks a bit about them:

It’s now up to ten times faster, and it uses much more historical data, so you get more accurate estimates of future availability. The new system allows us to quickly add new features in the future. We’ve already launched one of these improvements: Forecasts now take account of frequency caps, meaning that the availability estimates are modified to account for the fact that you might only want to show an ad once to each user. And more new features are coming soon!

Forecasting also continues to be a major development focus of our DART for Publishers (DFP) ad serving platform for publishers with larger direct sales teams, part of the DoubleClick Revenue Center suite of publisher solutions. Google Ad Manager serves as an effective complement to DFP and the DoubleClick Revenue Center to provide solutions for publishers of all sizes.

When it comes to forecasting, you must check your available inventory. When you do this, you can confirm when ad impressions are available before you book them.

According to the Ad Manager Help Center, if the check inventory query confirms that ad inventory is available, you can export the inventory query data into a line item in a new order or an existing order. With check inventory queries, you can optimize your ad inventory and prevent overbooking or underselling.

How do you check your available inventory in the first place?

Google provides you with a step-by-step process on how to do that here. There is also plenty more information available on forecasting, such as adjustments and troubleshooting here and an Ad manager FAQ here.