To start the stream, run this command:

raspivid -o - -t 0 -w 1280 -h 720 -fps 30 -b 6000000 -g 50 | ffmpeg -re -ar 44100 -ac 2 -acodec pcm_s16le -f s16le -ac 2 -i /dev/zero -f h264 -i - -vcodec copy -acodec aac -ab 128k -g 50 -strict experimental -f flv rtmp://a.rtmp.youtube.com/live2/your youtube id find you session here" in above line in your youtube channel 

• -o - makes it write the video data to STDOUT so it gets piped into ffmpeg.
• -t 0 is how you make it record forever
• -vf -hf flips it horizontal and vertical so it looks correct
• -fps 30 sets frames per second to 30
• -b 6000000 - output bitrate limit. YouTube recommends 400-600kbps, this is 600kbps. Change this to save upload bandwidth at the expense of a lower quality video
• -re - tells ffmpeg to slow down the reading of the input to the native frame rate of the input (useful when live streaming)
• -ar 44100 -ac 2 -acodec pcm_s16le -f s16le -ac 2 -i /dev/zero and -acodec aac -ab 128k and -strict experimental - adds a fake audio channel filled with zeroes (silence). YouTube rejects streams without an audio channel. You can also change the input device to a microphone if you want (I haven't done this yet)
• -g 50 adds a keyframe every 50 frames. Feel free to tweak, its one of those tradeoff variables.
• -f h264 and -f flv tells ffmpeg it's receiving h264 input and that you should mux it into flv output (flv is the container format that works with YouTube. Others might work but I haven't tried them)
• by not specifying -w and -h to raspivid we get the full 1920x1080 resolution (aka 1080p). You can specify these if you want a lower resolution