Adjust the video output settings on the devices connected to your Panasonic HDTV. To view 1080p programming, enter into the service menu on your external device and change "Video Output" to the appropriate resolution. By default, the Panasonic TV is always displaying content in the resolution provided for it by the external device. If your cable box is set to output in "1080i," for example, your TV will display content in 1080i.
Use HDMI cables to connect devices whenever possible. Just because your Panasonic TV is capable of displaying content in 1080 doesn't mean its receiving a 1080 HD signal. HDMI cables allow the TV to receive content from the external device completely uncompressed and in HD. If you used analog cables (like a yellow RCA video cable) to connect a device to your TV, you would see a standard definition picture even though both the TV and the device are capable of displaying high definition.
Connect your Panasonic HDTV properly to your receiver. HDMI cables, for example, are capable of transmitting audio and video over one cable. That doesn't mean that all receivers are capable of the same thing. Some receivers require two HDMI cables -- one for audio and one for video -- by design. If you're receiving audio and not video, or vice versa, and your HDTV is connected to a receiver, check the manual to see if the unit is capable of sending both signals over one cable.