If you want a lot of subscribers, NEVER ASK.
Just make good content that people would subscribe for and then spread the word about it as much as possible.
Try to have something different about your channel. For example, if you're doing an LP, try to put a spin on the way you commentate and play. NintendoLP is probably one of the best LP-ers I know. He's always happy and charming, and he's a hard worker. He thanks everyone for everything. If you post a comment on his video, he'll say on your channel "thanks for that comment on <video>!" So, if you put a spin on it and try hard, you'll get somewhere.