So you have done the hard part and got them to subscribe now its keeping them loyal and coming back for more.
Make your videos interesting, get in to a timetable find out when is the best time and day to do a video and stick to it do a video i.e every week on a Tuesday.
See if a fan knows there will be a video on that day at a set time they will return trust me tried and tested.
Work in advanced and have a few weeks videos always done and tease your fans(give a preview of next weeks show but keep it short and sweet) and they will watch next weeks video.
To bag more subs tell them to subscribe its really that easy, if your videos hit a funny spot or give good information the subs will come no probs.
Now not all people who sub use YouTube a lot so you need to show them you are a network and use Facebook twitter blogs or what ever you have.
Ask friends to share your posts about videos reach a bigger range of people coz lets face it each and everyone who sees your video could be a sub.
Use the tags in videos and if you do it right you will build a fan base that will stay for years, unless we get invaded by aliens that are hiding behind the moon and they want us has food :O OMG run!! (sorry about that i get carried away).
Hope this helps you

also make your channel look good and clean don't clutter it, people want it right here and now not have to look for it.