Using our YouTube Sub Button is an effective way to get your channel seen, it can also lead to more subscribers as you mentioned.
A way which I don't like seeing is this; 'drop and runs', something which we believe is irresponsible and shouldn't be done by a member of Freedom! - Essentially it is making a post on a forum with your channel link and asking for subs and then never contributing to the forum again.
Locate your YouTube URL, for example; youtube.com/channel/UCCMcOx2-vQGX4st4b5BmFIg
The final characters (after the channel/) is your Channel ID. - UCCMcOx2-vQGX4st4b5BmFIg
Once you have your Channel ID, put it in the box and save, and it should work.