Other than creating engaging content, keep uploading and what I've learned in some YT seminars is audience engagement at the end of the video such as "Thank you for watching, you guys are great if you haven't subscribed yet please do" and also creating channel art that includes a 'Subscribe' button and a schedule for when you plan on uploading new videos (eg; Mon and Tues); these are really the most preferred of ways. Naturally you can also do contests (which are pin pointed at subs only) and also find more carrots to put on the stick; external subscriber-ship can be garnered via your own website, instagram, facebook, twitter and other social outlets where you can request without much forms of spam. Just my two cents ☺