I use several different methods myself and I'm wondering what methods you personally use to gain subscribers.
Just have something that people would want to see. Something eye-popping, jaw dropping, etc. Actually, have several of these types of videos, because people will normally subscribe after they view not just one, but two, three, or more of your videos. And make sure you deliver what you say your video is about. If your title/description say it's about XYZ, then make sure the video is about just that.
Mine seem to be coming in naturally, I'd be interested to know of any techniques that can actively encourage more new subscribers though...
Actually, have several of these types of videos, because people will normally subscribe after they view not just one, but two, three, or more of your videos.