Well, add relevant titles and descriptions, respond to comments, add friends on youtube.
Try to find the channels that provide the same videos (or at least if they have the same target as yours) and watch what they do for promotion. You also need to engage social aspect as well.
But the best thing to get the video viral is to make it fun, useful, surprising and unique. You can also watch popular viral videos to find out what was their, you know, THING. The most popular viral videos got their popularity without special promotion, just because people shared them. So make your videos sharable, so people will distribute it among friends.
Very often viral videos got promoted through some digital agencies or famous digital people that noticed potential viral content and shared them through their web accounts. Try to inform these people about your channel, they are likely to mention you on facebook or twitter.