Greater Dynamic Custom Form Functionality

We have very complex forms and need the ability to program what items within a list will display based on a previous field selection. For example, if Video is selected as the Media Type, the Sub-Media Type field would only display: B-Roll, Cinema, Digital Video, In-Flight Video, In-Room Video, National TV, Spot TV. Whereas if Email is the Media Type, None would be the only option displayed in the Sub-Media Type field. By adding this functionality, Users are less likely to select the wrong value. GIGO!