Video is one of the worst offenders when it comes to data usage. It doesn't matter if you're streaming videos from YouTube or video chatting. It will really tax your data plan.
One minute of YouTube-quality video uses about two megabytes of data. Watch a half-hour television show daily, and you could exceed your data plan. Don't let your smartphone become your substitute for television. And save the videos for Wi-Fi whenever possible.
Streaming audio uses much less data than video. However, it is more dangerous. You listen to music when you're working or taking a run. It's easy to lose track of how long you're listening.
Suppose you listen to two hours daily. You'll reach your monthly data limit. If you must listen to streaming music, stick to Wi-Fi connections. Of course, smartphones offer ample storage for music files. Load your own music and refresh the selection periodically.