You guessed it; the easiest way to attract more subscribers is by consistently adding more content to your channel on a regular basis. The more you have out there in the sea of YouTube videos, the more likely someone is to run across one of them and find your channel.
You will also want to set a schedule for when you release your videos. If people see you are adding new content on a regularly scheduled basis, they are much more likely to subscribe then if they see you haven't added anything in a while and when you did it was sporadic.
Interact with Viewers
Nothing builds a sense of community better then a content creator who responds and speaks with their audience. If someone comments on your video be sure to thank him or her. If a conversation breaks out in the comment section relating to what was posted, chime in and give your two cents. Even if the comments are less than friendly, thanks them for giving it a watch and ask for some constructive criticism (except if they're trolling of course). Try ending your video with a call to the viewers asking their thoughts on something or even an idea for your next video. Getting your viewers involved will go a long way in building a relationship with them that will keep them around for a long time and spread the good word of mount about you.
Take a moment to research similar channels to yours that are already successful. Once you've found a few, compose a friendly message explaining a little about your channel and what you do. You may then want to suggest exchanging places on each other's featured lists. Doing this will show a link to their channel from your channel and vice versa. Now this is a big choice for another YouTuber to make, and you may want to wait until you have a decent video library built up before you go around asking people. If they see you are committed and pumping out content regularly, they will most likely accept your invitation and your channel will gain that much exposure.
Another huge benefit of exchanging featured spots with someone in your same field is that you will be displaying yourself right to your targeted audience. If you were to create a let's play channel for Dead Space and exchanged featured spots with a well known YouTube knitter, chances are not many of their viewers would want to check your channel out.
Asking for Subs
The line between asking genuinely for people to sub to your channel and coming off as begging for them to is a fine one. There are a couple different ways for asking viewers to sub, all of which should be placed at the end of your video. Starting off with a plea to subscribe to your channel is a terrible way to start off a video that is supposed to be entertaining/informative. Try to live by the golden rule of the Internet, never ask people for something unless you have given them something of value first (your video in this case).
After your main content has run it's course, take a moment to kindle ask the viewer to consider subscribing to your channel if they enjoyed your video.