I mostly shout out people I know or are related to me in some way. Like 'I was talking to 'so and so' and I think that...' or 'i saw so and so's video and inspired me to do...' or like 'omg so and so was right, that movie did suck!'. I ocassionaly do videos that are about shout outs with the links to the video and all that, but they're my fellow youtube friends or people I always watch.
I kinda prefer shout outs when they're natural, someone likes you or your videos that much to drop your name. It's way better than begging for one, cause when they do it, it's not a big deal... they're just doing it cause you asked them to.
One of the best shoutouts I ever got was last year or so when we asked people to drop their top 5 favorite moments of our show. This (at the time) random dude that I never heard of actually made a video about his top 5. We're good friends now, but at the time it blew my mind that this guy took the time out to make a whole video out of his choices.