If you have good quality videos, they will come. Sometimes you hit a plateau and think you aren't going to get any more, then next thing you've got an influx. There are websites that do sub for sub, or sell them, however, they are just a number, not fans, not someone who will comment on your videos, or give it a like, nor be keen to check out the next video you publish, also they can cause you get get stung for breach or terms with YouTube. Also it looks obvious you have fake subs if a channel has a large amount of subs, and very little views.
Make yourself known, get yourself seen, best thing I've found is to interact with viewers or other videos, I've asked questions on videos or left a comment on a sweet video and found that user ended up subbing me, simply because they got curious and went through to check out my channel from my comment.
Mostly, don't do it for subs. They are exiting to see increase. But ultimately, do it because you enjoy making videos and sharing them.