For me, it's mini-deadlines and a very consistent schedule. I know exactly what I need to have finished by the end of each day to get an episode done on time, and will just keep going until I get there. Storyboards and sketches need to be done in a day-and-a-half. Lineart and flats need to be done in a day and a half. Colours need to be finished after three days.
That prevents me from ever floundering around with no clue what I want to draw, which was what happened for most of last year. Making a webcomic gives me direction, and it turns out direction was exactly what I needed. I've drawn more in the past five months than I think I have in the past five years.
I don't get bored with the project because I absolutely love the story, so that's helpful too. I may wind up burning low a little in the future; this week was the first week where I've really been like "Uuuugh, I don't wanna work," but that's where the structure and mini-deadlines jump in to save me. I know what I need to do, and I can do it, regardless of whether or not I really want to that day. (And once I had the sketches done, I bounced right back!)
