What's helped me a lot is: making sure my scripts are in order, making sure my thumbnails are in order, not skimping out on doing sketch pages, AND, not zooming in too much with digital art. (This is a huge one in terms of time...) Really, just organization and strategies to keep art fast and fun.
Alternatively, setting some deadlines or "challenges" for yourself (those these should never be too serious or else they can become stressful) but for example, saying, I want to finish x chapter by x time can sometimes be a good motivator. (That's how I developed by 60+ page buffer. I focused on book releases rather than updates online and ended up with lots of pages, hehe.) This isn't for everyone though, just something I found personally works for me because I love setting a goal and then getting it. >: )
Good luck! Everyone else here seems to have a ton of good ideas about this too 
